Aimed at the process of laser rapid prototyping with inner coaxial powder feeding, the finite element model on the transient temperature field of the molten pool is built and the transient temperature field of the molten pool under the laser with variable duty ratio is simulated numerically. The results show that the duty ratio of the laser affects the shape, dimension and power distribution of the molten pool. With the processing parameters: laser power (P)=3000W, outer diameter(d2)=4mm, scanning speed(V=5mm/s), the ideal value of ratio duty(K) is from 0.35 to 0.6. The shape of the molten pool is saddle shape or crescent. The position of the molten pool falls behind the center of the laser beam with the deviation to the both sides. Through shooting the molten pool with the high speed CCD camera, we find the experiments show that the images are close to the numerical simulation nephograms and then demonstrate that the numerical simulations are right.