The traditional bentonite mud can hardly meet the engineering requirements because of serious slurry leakage， frequent trench side collapse， and highly increased construction cost. On the basis of theoretical analysis， experimental research was carried out to develop a new type of trench side protection mud by comprehensive modification of natural clay mud to solve the problem of trench side collapse in sand， gravel， and boulder layers. The basic modifier CN-1 was determined through the optimization test to improve its suspension stability. On this basis， the adhesive lifting agent （CG-14S） and the cooperative plugging agent （DFD-2） were further obtained through optimization to improve its comprehensive performance index. The optimized formula was determined by orthogonal test， and the performance and cost evaluation was carried out. Field tests were conducted， and a modified clay mud for trench side protection was finally developed. It is made up of 15% natural clay+0.2%CN-1+0.4‰CG-14S+0.5‰DFD-2， and can be used in the construction of the diaphragm wall in gravel and boulder layers. The research results provide an option for solving the problem of trench side collapse in the construction of the diaphragm wall in complex strata.