This paper presents the numerical investigation of ultrasonic guided waves for bolted lap joint Structural Health Monitoring (SHM), considering temperature variations. It first systematically discusses the mechanisms behind the linear and nonlinear ultrasonic techniques for bolt loosening detection. Afterwards, a general Finite Element Model (FEM) of a bolted structure was established to observe and verify the influence of the change of bolt pre-tightening force on the linear and nonlinear characteristics of ultrasonic guided waves. In order to further study the effect of temperature variation on the sensing signals, the structure is subjected to various levels of thermal loads. This study examines the temperature influence on both linear and nonlinear signal features, such as the transmitted wave energy and the nonlinearity of waveforms. Simulation results show that an increment in temperature can cause partial detachment of the interface between two lap joint components in the structure, resulting in a decrease in both the linear energy and the degree of nonlinear higher-order harmonics. The paper finishes with concluding remarks and suggestions for future work.