Abstract: A large area field immersion test with water pool of 110×70 m2 was conducted combining with ground treatment of the eleventh pump station using prewetting method.Building the pump station was a national key construction project of the Yellow River for irrigation in Ningxia province,China.The soil stratum of test site was self weight collapse loess with a thickness of 35 m,and the test was sustained for 251 days.It was shown that there were three distinct characteristics of the collapse deformation of the self weight collapse loess with big thickness(i.e.,more than 15 m)different from that with middle-small thickness(i.e.,less than 15 m):① The process of collapse settlement developing with immersion time consisted of 5 stages,i.e.,initial gentle change,abrupt increasing,smooth progress,fast enhancement after cutting off water,and slow variation.② The rate of collapse settlement varied with immersion time in such patterns:low,high,low and gentle;After cutting off the water supply,the rate of collapse settlement appeared with time in such fashions:large,small and gentle.③ The collapse settlement,the width of the crack and the level difference of two sides of the crack around the test site were much larger than the corresponding record of the similar tests in the past and in other regions of China.A criterion for stabilized collapse settlement and cutting off the water supply was proposed via analysis of the results of the field immersiontests in the past years and in other regions of China.The proposed criterion was that the rate of collapse settlement was not larger than 2 mm/d for 5 days in succession.The time limit and investment for the project was reduced because of adopting the criterion.The research results might be useful to similar projects and to revise “Code for building construction in collapsible loess regions”.