In order to detect the velocity and concentration field of metal power particles in laser cladding,an optical system is built based on a 100-mW diode laser. A combined lens is designed to collimate the beam of laser,after the elliptic spot corrected by a prism into a circular one. At the last laser beam is extended into uniform sheet laser by a cylindrical mirror. Laser physical model after scattering by spherical powder particles is constructed from Mie scattering theory . Laser intensity distribution is plotted by mathematical software when it is irradiated by green and red light . The research result shows that scattering intensity of green light is two times of red's under the same condition and sub-maximum intensity of 532-nm light is appeared around 20°. So 532-nm diode laser is selected as the light source. Pictures are much better when the angle between charge coupled device (CCD) camera and incident direction is 20°.It provides the theory for a cheap and portable powder flow field detecting system.