A device for drop-freezing experiment was improved,which consists of the cold-cavity,thermometer,controller of temperature,recorder of signals,cold sink,and environment box.Before the drop-freezing experiment,the water sample to be tested was evenly scattered as several dozens of drops in the cavity over the cold board.The temperature of drops can decrease linearly under auto-control and the drops freeze in succession with decreasing temperature.An array of thermocouple is used to detect the latent heat released by freezing drops and a "pulse" signal of the freezing event can be revealed in real time on a pen-andink recorder.After several times of drop-freezing experiments for certain water samples,the freezing temperature spectrum can be obtained.According to the method given by Vali,the concentration of the freezing nuclei for this water sample can be calculated.Using this device,drop-freezing experiments were carried out for several kinds of precipitation water samples in Beijing.The results show that there is obvious deference in temperature spectra of freezing nuclei for different precipitation samples.It will be hopefully used as a new criterion for effectiveness evaluation of weather modification operation.