The stability of trench face of diaphragm wall is the most critical question in diaphragm wall construction quality and construction safety; and collapse accidents of trench face occur now and then due to inappropriate protection measure. The paper first analyzes the instability mechanism and form of trench face, and sums up both the disadvantages and advantages of every kind of anti-instability reinforcement measures for trench face. The paper also puts forward the suggestion value of rational reinforcement width and the principle of certainty of rational depth of deep mixing pile. Then, it summarizes the application conditions of every kind of stability safety factor calculation method of trench face under the terms of slurry protection, and discusses the rationality of every kind of safety factor value, and gives the recommended values. Finally, it provides the stability safety factor calculation method and decision criterion under the terms of mixing pile reinforcement. Through a number of engineering practices, the reliability of stability safety factor calculation method and stability decision criterion of trench face and both the rationality of depth as well as width under the terms of mixing pile reinforcement have been verified.