The failure mode of cracks in the mounting hole area under the input conditions of static and dynamic loads in the various conditions of working environment is analyzed, when brittle plastic parts are installed with threaded connections. By combining theoretical calculations and structural simulation analysis, a method for strength calculation and check as well as a calculation method for limit value of mounting and fastening torque are proposed. The methods have been effectively verified through mechanical environment testing. And the improved design measures of installation and fastening are given.