The powder diameter affects the heat generation and further influences the quality of weldments when the MZA powder is used as laser-absorbents to carry out laser transmission welding (LTW). It is difficult to measure the temperature due to its extremely short duration. A mathematical model considering the multiple reflections of metal powder is established to predict heat generation. The mathematical model is verified and then used to describe the influence of line energy density and powder diameter on the temperature profile of the LTW process. The results demonstrate that the temperature from experiments and simulation are consistent. Therefore, the established model is suitable to predict the heat generation of metal powder laser-absorbents in the LTW process.