Characteristics of nitrogen mineralization in different soil layers were studied in a seven-year-old protected tomato field irrigated in three ways, infiltration irrigation, drip irrigation and furrow irrigation, using the long-term intermittent leaching aerobic incubation method (Stanford and Smith). In infiltration irrigation, transpiration semi-soft pipes were laid down 30 cm down in the soil. The experiment was managed in the same way as in the local farming production in irrigation, fertilization and field management. When the water suction in the soil reached 40 kPa at 20 cm in depth, irrigation began. The irrigation rate of infiltration irrigation and drip irrigation was half the rate of furrow irrigation in each event. Results show that mineralized N decreased with soil depth in content. Based on the correlation curve of cumulative amount of mineralized N and incubation time, the soil within the depth of 0～50 cm could be divided into three layers, i.e 0～20, 20～40 and 40～50 cm in both infiltration irrigation and drip irrigation, and 0～30, 30～40 and 40～50 cm in furrow irrigation. A two-pool model was used to fit N mineralization in different layers. Comparison between irrigation methods showed that in the 0～10 cm soil layer, the content of mineralizable N (N₁) followed the order of drip irrigation > furrow irrigation > infiltration irrigation, and that drip irrigation was the highest in mineralization rate ( k₁) of mineralizable organic N, suggesting that drip irrigation facilitates formation of mineralizable organic N in the surface layer of the soil. Compared with infiltration irrigation and furrow irrigation, long-term drip irrigation is more conducive to improving quality of soil organic nitrogen in protected fields.