Based on the established three-dimensional rough surface model, the transient finite element model and the thermal contact model, the effects of laser power, welding speed and clamp pressure on the contact thermal conductivity of laser transmission welding of polycarbonate are studied, and the comparison between simulated and experimental results is also conducted. The study results show that, with the increase of the welding speed, the contact thermal conductivity decreases and the decrease becomes slow. With the increase of laser power, the contact thermal conductivity increases and the increase becomes slow. When the clamp pressure is within the range of 0-0.25 MPa, the contact thermal conductivity increases with the increase of the laser power. A relatively high contact thermal conductivity can be achieved when the conditions of high laser power, low welding speed and high clamp pressure are satisfied. The contact thermal conductivity can be well predicted based on the built mathematical models.