To provide guidance for structural design and optimization of the functional sub in the drill string， the model of the mud pulser sub in Type 675 instrument string is simplified and the finite element analysis model is created in the case of the extreme working condition under the combined load of compression， bend and torsion. Considering the structural characteristics of the mud pulser sub in Type 675 instrument string， different mesh division methods are adopted for different sections of the mud pulser sub. Fixtures are added on both sides of the model for composite loading， while equivalent loading is applied for the bend load in the combination of compression， bend and torsion load by calculating the maximum deflection under the limit dogleg. The simulation results show that under the combined load of compression， bend and torsion， the stress concentration phenomenon mainly occurs in the drill collar and the electronic framework， and weak positions such as openings， grooves. Based on the analysis results， suggestion is put forward on the structural improvement of the mud pulser sub in 675 instrument string. The analysis method and design experience presented in this paper can provide reference for the design and optimization of the functional sub of the same kind.