For the construction of diaphragm retaining wall trenching in sand-bearing soft soil layer, the construction quality will be affected by the collapse of side wall of trench. Water loss reducing, appropriate slurry viscosity increasing and collapse prevention are the key for wall protection in trenching. According to the geological conditions of a diaphragm retaining wall trenching engineering in Ningbo, partially hydrolyzed polyacrylamide (PHP), calcium polyacrylonitrile (CPAN) and potassium nitryl humate (NKHm) were taken as additives; the optional ratio of slurry performance was ascertained by the orthogonal test. The test results show that when using PHP, the amount of water loss is small with thin mud cake; CPAN and NKHm have evident effects on fluid loss control with moderate viscosity. The test result is applied in the engineering practice and the good effect of wall protection is received.