Probe of Cloud Droplet (CDP) based on forward scattering has the characteristics of high efficiency, simple structure, and it is the main instrument for small cloud particle (less than 50μm in diameter) detection. A CDP based on forward scattering was constructed in this paper. Calibration experiment and the measurement of the field of depth were conducted. To measure the depth of field, a pinhole adhered to a rotating disk which is fixed to multi-dimensional optical table is used. The pinhole can be seen as a particle appeared in the detection area periodically, as it moved across the laser beam, the depth of the field can be measured. Standard particles with known size were used to calibrate the system and Mie theory was used to calculate the intensity of forward scattering signal. In the end, combining the experiment results and the theoretical calculations, the response curve which can be used to inverse the size of cloud droplet was obtained.