Abstract: Aiming at current problems of traffic flow investigation in multi-roads crossing intersection, this article introduces a new way to investigate traffic flow. This method combines car licence and mathematical theory and uses different numbers of all kinds of types to random sample the cars in each approach of five-roads crossing intersection. Then the practical result is calculated with sampleanalysed result, which uses car license to determine car types and numbers in each approach and computes traffic flow towards every direction by mathematical theory. At last an example is put forward to prove this new idea is practicable.