China now faces the same challenges as advanced countries in increasing teachers’ quality after the object of meeting the demand of qualified teachers being reached.This article addresses a slowing down marginal increasing trend since 200506 based on a time serial study on teachers’size，qualification and ages during 1980—2011.The tendency is explained as a decreasing return on education quality of human capital investments on teaching force in term of expansion，qualification and teaching experiences.Two conclusions are then founded：(1)Increasing returns on education quality can only be reached by new investment ways，named as ‘learning by doing’ in workplaces to solve practical problems.(2)Organizational learning instead of teacher's individual's learning should be facilitated and inspired.In order to improve the utility of “Learning by doing”， the government need to reform the foundermental system in public education and evaluate education quality.