Abstract: A radioactive detection system for water on the vehicle based on NaI(TI) detector was developed. In order to optimize the structure of the detection system and to save the cost, the Monte Carlo code Geant4 was used to simulate the detection efficiency and the peak value of the detector when the holders with different volume were used in the detection system. After efficiency calibration, the detection efficiency of actual measurement for all-around peak count were similar with the results from Monte Carlo simulation results. To further understand the relationship between the relative error and the performance parameters of the detector, such as detection time, detection limit and the activity, the 180 Bq/L ¹³⁷Cs standard solution that was prepared by the State Administration of Science. Technology and Industry for National Defence. PRC was placed in detection system on the vehicle for experiment. The detection performance of system satisfied the requirement of subject index requirement. In the event of a nuclear accident, we need quickly reach the designated test area and conduct rapid quantitative testing which can provide the basis for the rapid response of the management department.