A method and device for monitoring the position of radioactive materials in vehicles relates to a technical field of monitoring of radioactive materials. The method comprises following steps: powering on a monitoring device, after initializing, the device automatically enters into background mode to acquire and process gamma and neutron detection data so as to obtain and update a background count rate in real time, and an image surveillance system enters into surveillance state; occupancy detector detects that a vehicle enters into monitoring channel, meanwhile the monitoring device automatically enters into occupancy mode, and performs data acquisition process for gamma and neutron detection under the occupancy mode in accordance with the time required by an alarm algorithm, so as to obtain respective total count rates; speed detector detects the times T1 and T2 for the vehicle when it reaches two points with a distance L, and then calculates out the vehicle's speed V; obtaining an alarm threshold by using a special algorithm on the basis of alarming algorithm, particularly, the background count rate updated in real time, and sending an alarm signal when the total count rate exceeds the alarm threshold; transmitting the alarm signal to the image surveillance system via a control interface to notify it of starting to record, meanwhile sending an alarm command to an audio and visual alarm system to inform it of giving out an alarm; with the help of the results of radioactivity detection, speed detection and image capturing, a local computer specifies the position where the radioactive material(s) locate(s) with methods of horizontal and vertical positioning. The present invention can easily and quickly specify the very vehicle and the exact position the radioactive material(s) locate(s) so that the radioactive material(s) can be conveniently isolated and processed subsequently. In this way, time spent on detection has been greatly reduced and lots of human power has been saved.