In order to overcome the shortcomings of traditional particle measurement, a new method for the detection of small and single particles through laser feedback phenomenon is presented. This method is based on the laser feedback caused by radiation scattered back from a moving particle in the external cavity of the laser. The parameters of the single particle, such as the diameter, velocity, and quantity, can be measured and calculated from the change in output laser power. In the experiment, the confocal external cavity composed of a concave reflector and a positive lens is designed. This device is able to obtain the corresponding variety curve of standard particles passing through the confocal area, and then the parameters of the particles can be measured and calculated by combining the experimental data and standard curves. Experimental results show that this method is an easily operated and reliable way for particle detection. The measurement ranges from 0.2 to 2000 mm, resolution is 0.2 mm, and measurement error is within 2%. This device may have wide application in areas such as atmosphere particle detection and calibration of a single particle producer.