The invention discloses a method and system for measuring the fresh air volume in a space to be detected. The method includes the steps of obtaining the volume data V of the space to be detected; detecting and continuously recording particulate matter concentration data Cout outside the space to be detected, detecting and continuously recording particulate matter concentration data C in the space to be detected and performing calculation and processing according to the data volume V, the particulate matter concentration data Cout outside the space to be detected and the particulate matter concentration data C in the space to be detected to obtain fresh air volume data Q. By the adoption of the method and the system for measuring the fresh air volume in the space to be detected, the fresh air volume in the space to be detected is obtained by directly detecting the particulate matter inside and outside the space to be detected and then performing calculation, a detection method is simplified, and the detection process is more convenient.