In recent years, 200/400 Gbit/s transceiver in the form of Quad Small Form Factor Pluggable-Double Density (QSFP-DD) is favored by the market due to their relatively low power consumption and small size for high-density deployment. However, the deterioration of heat dissipation caused by high speed and small size limits the application of QSFP-DD optical modules. In this paper, the finite element method is used to conduct thermal modeling and simulation of QSFP-DD module, and the internal temperature field of 200 Gbit/s QSFP-DD Long Range 4(LR4) optical module in high temperature environment is studied. The effect of thermal pad on improving the internal heat dissipation of the module is verified. The performance of the module in high temperature environment is measured. It provides a reference for the design and application of QSFP-DD transceiver module.