Forward scattering spectrometer probe (FSSP) is one of the most important instruments in the weather modification activities. A prototype of a FSSP for routine observation of cloud droplet was presented in this paper. In this system, a single-mode 685 nm diode laser with single-mode fiber output coupling was used as a transmitter, a square pinhole combining with a 4f optical system were used to obtain homogeneous illumination, the illuminating laser was abandoned by the hole on the 45°reflective mirror, and the depth of field of this system was controlled by a 200 µm pinhole placed on the focal plane of the collecting system. Both the scheme of optical design and the experiments were presented in this paper. The experiments showed, the laser beam was close to flat-top, the illuminating laser can be abandoned efficiently, the scheme of controlling the depth of field was reasonable. Simulation experiments using pinholes of different sizes indicated that the prototype had the ability to detect cloud droplets.