Abstract: This paper used basin pot culture with plastic film method to study the dynamic characteristics of soil nutrients and soil microbial biomass during 120 d decomposition of different proportions of wheat and corn straws with different decomposing agents (F1: organic waste fermentation bacteria; F2: microbial agent; F3: "Manyuanchun" bio-fermentation). The results showed that during 120 d composition of wheat and corn straws, the increasing rates of soil organic matter, available nitrogen and total nitrogen in each treatment was rapid at the early stage and slow at the latter stage. The changes of increasing rates of soil phosphorus and potassium followed rapid-slow-rapid-slow trend. Throughout the trial stage, the soil microbial biomass carbon (SMBC) content of wheat treatments were increased first and then decreased. However, soil microbial biomass nitrogen (SMBN) content showed an alternating changing trend. Change in SMBC content of corn straw treatments was quite different from that of wheat treatments after 100 d composition. Treatments with decomposing agents improved soil nutrients, SMBC and SMBN contents, showed significant effects on soil fertility improvement. Among three decomposing agents, F3 (containing bacillus, filamentous fungi, actinomycetes and yeasts with special features) was the best in enhancing soil nutrients contents. The performance of different straw decomposing agents for wheat and corn straws were wheat > corn for F1, wheat ≥ corn for F2 and wheat < corn for F3. Therefore, F1 (training by multiple aerobic complex strains with a strong ability to decompose the cellulose, hemicelluloses, lignin and other organic ingredients of organic waste) and F3 most promoted wheat and corn straw decomposition, respectively. F2 (rich in microbial flora which decomposes cellulose, hemicelluloses, lignin and other bio-organic substances) equally promoted wheat and corn straw decomposition. For different decomposing agents, the order of SMBC and SMBN contents in wheat treatments was F2 > F3 > F1. The orders SMBC and SMBN in corn treatments were F2 > F3 ≈ F1 and F3 > F2 ≈ F1. SMBC in each corn straw treatment was greater than that in wheat straw treatments, for SMBN it was inverse. This was consistent with the amount of C/N ratio of the straws, the greater the C/N ratio the greater the SMBC content and the smaller the SMBN content.