As not much correlation was observed between rice N uptake in Treatment N0 (No N fertilizer applied) and N mineralization rate measured with the traditional In situ incubation method, modification was made of this method in an attempt to improve its accuracy in measuring soil N supplying capacity. The traditional one is a continuous incubation method, which goes like this: collect some soil from a paddy field before seedling transplanting, mix it with water at 1:1 in volume, pack the mixture in ziplock bags, put the bags back into the field for In situ incubation, and then sample the soil in the bag once every 30 days for measurement of inorganic N. The modified one adopts phase incubation and goes almost the same as the traditional one in preparation for incubation, and then put the bags in centrifuge tubes full of water and the tubes back into the field for incubationIn situ and sample the soil in the bag for analysis of inorganic N once every 30 days, while removing the ziglock bags of the last phase of incubation. Results of the 2013-2015 experiment show that with the incubation going on, mineralized N content rose first and fell as was measured with the traditional method. Compared with the maximum value, cumulative mineralized N dropped by 6.7%~28.6%. However, the modified one did not see any decline. So the measurement using the traditional method was 30.0%~67.7%（p <0.05）lower than that using the modified one. Duration of the incubation is a major factor affecting the content of mineralized N. When incubation went on continuously over 40 days, it might pose a risk of inhibiting N mineralization. So, each incubation should not last too long or over 40 days. The content of mineralized N measured with the modified method was found to be closely and positively related to rice N uptake measured in Treatment N0, with R² being 0.621 (p <0.01). Therefore, it is suggested that when measuring soil mineralized N in paddy field, measurements using the modified one be cited as an indicator to evaluate soil N supplying capacity.