Probe of cloud droplet (CDP) based on forward scattering is one of the most important instruments for small cloud particles, smaller than 50 μm in diameter, measurement because of its high efficiency and simple structure. A CDP based on forward scattering is constructed. To measure the depth of field, a pinhole adhered to a rotating disk which is fixed to multi-dimensional optical table is used to simulated as a particle appeared in the detection area periodically, as it moved along the laser beam, the depth of the field can be measured. Standard particles with size of 5, 10 and 30 μm are used to calibrate the system and the response curve to standard particles is obtained. Mie theory is used to study the corresponding relation between the system′s responses to standard particles and cloud droplet. The response curve to cloud droplets is obtained to measure the sizes of cloud droplets.