Selenium-rich rape Selenium Ziyuan No.1 was used as green manure to study the effects of different amounts of green manure returned to the field on the release characteristics of available selenium in acidic paddy soil in southern China, and to analyze the absorption and transformation of selenium in rice, so as to provide a theoretical basis for planting natural selenium-rich rice in acidic areas of southern China. Six treatments with different amounts of selenium-enriched rapeseed returning (0, 5, 10, 15, 20, and 25t/hm²) were set up. Two rice varieties (selenium-rich rice variety Meixiangzhan 2 and common rice variety Zhongguangxiang 1) were selected. The results showed that (1) with the increase of selenium-rich rapeseed returning amount, the available selenium in soil showed an increasing trend. Over time, soil available selenium showed a significant increasing trend, and the content of soil available selenium reached the maximum at tillering stage, and then decreased. (2) For selenium-rich varieties, when the amount of selenium-rich rapeseed returned to the field was less than 15t/hm², the selenium content in rice grains increased significantly with the increase of the amount of selenium-rich rapeseed returned to the field, then remained basically stable. For conventional varieties, with the increase of the amount of selenium-rich rapeseed returned to the field, the selenium content of rice grains showed an increasing trend, but the overall selenium content was much lower than that of selenium-rich variety. (3) With the increase of the amount of rapeseed returned to the field, the rice yield had an increasing trend, but the maximum rice yields appeared when the amount of selenium-rich rapeseed returned to the field was 15t/hm². Therefore, Se-enriched rape returning could promote the release of available selenium in soil and the enrichment of selenium in rice plants, and significantly increase the selenium content in rice. According to the selenium content and yield of rice, it is suggested that the selenium-rich rice variety Meixiangzhan 2 was chosen and the amount of Se-rich rape returning is 15t/hm².