【Objective】To ameliorate salt-affected soil in the Yellow River Delta for productive use is of great social and economic significance to the region. Trials have been conducted to use leonardite as a soil amendment for the purpose, but failed to get consistent results and probe in-depth into the causes. This research aimed to assess changes in salt composition of the soil solution in the salt-affected soil applied with leonardite, and reveal processes and mechanisms of the changes. 【Method】Three topsoil samples (S1, S2 and S3), different in salt contents (1.0, 7.5, and 35.3 mg•g^-1, respectively), were collected from a farmland in the Yellow River Delta. After being air dried and gently ground to pass a 100-mesh sieve, the samples were mixed, separately, with leonardite at 0%, 1%, 3%, and 5% in dosage and each treatment had two replicates. Then the mixtures were prepared separately into suspensions (solid: water = 1g: 5ml), which were shaken at 300 r•min^-1 for 7 d and then centrifuged at 3 000 r•min^-1 for 10 min. The obtained supernatants (or soil extracts) went through a 0.45-μm filter and got ready for analysis of Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl^-, NO₃^-, and SO₄^2- with ion chromatography. The leonardite used in the experiment was digested by HNO₃-HClO₄-HF for analysis of Ca²⁺, Mg²⁺, K⁺, and Na⁺, and its carboxyl and phenolic hydroxyl groups were determined with the titration method of the International Humic Substances Society. 【Result】The leonardite was found to contain Na⁺ (1.66 mg•g^-1), K⁺ (1.07 mg•g^-1), Ca²⁺ (19.97 mg•g^-1), Mg²⁺（1.60 mg•g^-1), carboxyl group (2.74 mol•kg^-1) and phenolic hydroxyl group (1.63 mol•kg^-1). In the soil solution, sodium adsorption ratio (SAR) decreased with leonardite increasing in spiking dosage, which was attributed to Ca²⁺ release from and Na⁺ adsorption to leonardite. In the treatment of adding 5% leonardite to soil S2 SAR reduced from 7.81 to 6.61. In analogy to SAR, Cl^-/SO₄^2- molar ratio was proposed as a new indicator to reflect changes in anion compositions. It decreased from 10.20 to 8.25 in the treatment, which was a result of SO₄^2- release from and/or Cl^- retention by leonardite. The reduction of SAR and Cl^-/SO₄^2- ratio suggests that leonardite could lessen the toxic effects of Na⁺ and Cl^- on plants. 【Conclusion】With Ca²⁺ and abundant carboxyl groups it contains, leonardite could alter ion compositions in soil solution via: 1) ion exchange between Ca²⁺ in leonardite and Na⁺ in soil solution, reducing Na⁺ concentration in the solution and its harmful effect; 2) replacement of Na⁺ adsorbed on soil colloids by Ca²⁺ released from leonardite, favoring formation of soil aggregates and leaching of Na⁺；and 3）formation of complexes of Na⁺ in solution with organic substances dissolved from leonardite, reducing Na⁺ activity and toxicity. It is, therefore, proposed that SAR and Cl^-/SO₄^2- ratio be tested as indicators for fast laboratory assessment of suitability of soil amendments for use in salt-affected soils, which would help land managers choose proper soil amendments for use and avoid input of Na-rich organic materials, such as sodium humate, in salt affected soils.