When a colloidal particle carrying surface charge is dispersed in an aqueous solution, it adsorbs a large volume of ions, reverse in charge at its solid/liquid interface, thus forming an electric double layer (EDL), which plays a vital role in ion sorption and desorption at the solid/liquid interface of charged colloidal particles and in stability of the colloidal particle suspension and stability of protein system According to the theory of the EDL model, a shear layer exists between the shear plane where Zeta potential (ζ) exists in the electric double layer and the particle surface, and thickness of the shear layer is the distance between the shear plane and the particle surface. Thickness of the shear layer is not only an important physical parameter in the research on colloid and interface chemistry, but also closely related with some electro-chemical properties, such as electrodialysis, electrophoresis, streaming potential and sedimentation potential. Nowadays, researchers commonly use the colloidal particle double electro layer model and DLVO theory in their studies on thickness of the shear layer and hold that the shear layer is very close to the Stern layer or the outer Helmholtz layer; and as thick as the diameter of about 2 ~ 3 water molecules, about 0.5 nm, because they deem zeta potential approximate to the surface charge of colloidal particles. Meanwhile some researchers have figured out that the shear layer is 0.03 μm, 0.1 μm, < 0.25 μm or no more than 2 μm and believe that particle surface charge is infinite. And also some researchers have concluded through theoretical analysis that the shear layer is very closed to the Gouy plane in the electric double layer, however, its actual position is hard to determine. In light of the above described analyses, it is quite clear that the results the researchers obtained as to thickness of the shear layer vary sharply, mainly because it is very hard to measure accurately colloidal particle surface charge. In the colloid diffuse electro double layer theory, although it is still not clear where the particle shear layer actually is, it is certain that it lies in-between the outer Helmholts layer and the Gouy layer. Therefore, it is feasible to use the Gouy-Chapman potential distribution equation to describe the relationship between potential φ(x) of any point in the double electro layer and its corresponding point x. In this paper, from the Gouy-Chapman theory an equation was derived to calculate thickness of the shear layer (xS) in the single electrolyte system. Based on the soil colloid surface potential and zeta potential measured by zetaPlus, thickness of the shear layer (xS) was calculated. (1) Surface potentials and zeta potentials of colloidal particles in acidic, neutral and calcareous purplish soil samples are much higher in the 1:1 type electrolyte system than in the 2:1 type electrolyte system, and in the two single electrolyte systems, the variation of surface potential with concentration of the electrolyte is much sharper than that of zeta potential. (2) In the two single electrolyte systems, all the three purplish soils show the colloidal shear layer is quite far away from the Stern layer, but quite close to the Gouy layer in the diffuse double layer (DDL), and the shear layer is much thinner in the 2:1 type electrolyte system than in the in 1:1 type electrolyte system. (3) The shear layer gets thinner with rising concentration of the electrolyte in all the three soils. On different soil colloidal particles with surface charge, the shear layers differed significantly in thickness when concentration of the electrolyte is low (p<0.05), and not so significantly when concentration of the electrolyte is high (p>0.05). This study has not only brought forth a new theory and method for calculating thickness of the shear layer of soil colloidal particles in the single electrolyte system, but also provided some bases for enriching the theories of colloid diffuse double layer structure.