Due to the Zn bioavailability is low in northern calcareous soil，the Zn content of crops couldn’t meet the nutritional needs of humans.Previous field experiment showed that different green manures increased the afterculture wheat grain Zn content differently，and the wheat grain Zn content were positively correlated with green manure shoot Zn content.However，why green manure took up Zn differently and the soil Zn mobilization mechanism were still unclear.Field study，hydroponic culture and soil Zn extraction experiments were carried out to investigate the Zn content of green manure (black kidney bean，mung bean，sorghum×sudan Grass)，and the main components of their root extrudates and the effect on soil pH and extractable Zn2+ by main low molecular organic acid of root extrudates，with the purpose of unveiling differences in Zn uptake by different green manures and the soil Zn mobilizing mechanism of green manures.Results showed the total dissolved organic carbon，total low-molecular weight organic acid and the main components of the root extrudates of green manures were significantly different from each other，affecting the concentration of Zn in the shoot of green manure.The root secretion ability of black kidney bean was stronger than other two kinds of green manures，the total organic acid and dissolved organic carbon were both the highest among three different green manures and also black kidney bean had the highest shoot Zn content.With Zn application，green manure secreted higher amount of organic acid than those without Zinc application，resulting that the Zn content of green manure with Zn supply were higher than those of green manure without Zn supply.All four main organic acids could significantly decrease soil pH and increase soil soluble Zn2+.Among all the low molecular weight organic acids of root extrudates，oxalic acid secretion was the highest，followed by succinic acid and citric acid，and fumaric acid was lowest，and oxalic acid had the strongest effect.Compared with the mung bean and the sorghum×sudan grass，black kidney bean excreted the highest total organic acid，which might be the main mechanism explaining that black kidney bean can increase the soil available Zn and take up the most Zn.