Under the conditions of radius of the incident laser beam is much larger than the thermal diffusivity of the materials, the
model has been established with the heat transfer theory and the finite element method. Analyze has been made out on
the thermal interaction of long pulse laser and semiconductor Si and Ge. The influence of the radius of incident laser
beam on the size of melting area on sample has been considered. The relation between the rate of the max melting radius
on fused sample and the radius of incident laser beam Φ'/Φ and the laser fluence has been given out and discussed,
which is of importance to the research on the interaction mechanism of the laser beam and semiconductor material.