Abstract: 【Objective】This study aims to explore the influences of chopped straw length on mechanical properties of black soil. It will provide theoretical basis for compaction and restoration, erosion and protection of black soil.【Method】We chose typical black soil without reclamation(high organic matter and low clay content)and tillaged for 40 years(low organic matter and high clay content)as subject simples on Keshan Farm of the northwest of Heilongjiang Province in June 2016. The adding mass fraction of the straw was 1%, soil gravity water content was 20%, and the initial density was 1.1 g/cm³. We combined the indoor oedometer test and triaxial test, to obtain and analyze the compression index, rebound index, cohesion and internal friction by simulating different chopped straw lengths returning(1, 5, 15, 25 mm, soil treatment without straw), then investigated their effects on soil compression, rebound and shearing properties.【Result】 Straw returning could effectively improve the mechanical properties of black soil. The effect of straw returning on low organic matter high clay soil samples due to long term tillage were more significant with the increase of chopped straw length. With the chopped straw length increasing, the compression index and rebound index was negatively and positively correlated with straw length, respectively; especially when straw length was 15 mm, the compression index decreased to the minimum value, and at the same time rebound index significantly increased to the maximum value; or black soil is hard to be compressed, and most easily rebound to the natural state after compaction. Soil cohesion significantly increased with straw length increasing, leading to higher shear strength, but the increase amplitude was decreasing gradually.【Conclusion】Straw returning measure has significant positive effects on the mechanical characteristics of black soil, and the optimum straw length of simulated experimental results is 15 mm.