Pot experiments were carried out on the ¹⁵N-lablled soil after winter wheat to study the changes of soil microbial biomass C,N and soil fixed ammonium during rice growth. The results showed that soil microbial biomass C increased with the rice growing and reached the highest at the harvesting stage. The soil microbial biomass N decreased a little bit at the beginning of experiment and then rose to the highest at the tillering stage, after which it decreased again with the increase of N absorption by rice. The content of soil fixed ammonium changed little during the rice growing.some of the ¹⁵N fixed by soil clay minerals exchanged with the soil mineral N, showing that the fixed ammonium in last crop was also effective to rice at the second cropping.