The invention, which belongs to the field of road engineering survey and design, discloses an automatic calibration method for a road axle load spectrum detecting system. According to the method, theweight of a first axle of a 1127 type heavy-duty lorry among ten kinds of widely-used vehicles in road transportation is used as a reference weight; a detection result of the weight of the first axleof the heavy-duty lorry is compared with the reference weight by the system; and system parameters are corrected in real time by means of weighted correction, thereby realizing real-time automatic calibration of the axle load spectrum detection system. With the disclosed method, regular system calibration by using a specific vehicle is avoided, so that the convenience of the system usage is improved. Besides, real-time detection of the vehicle in normal driving on the road is realized; the state change of the detecting system is tracked in real time and the adjustment of system parameters is realized continuously and automatically by means of real-time correction, so that the test accuracy of the system is improved effectively. The method has the great promotion application value and can be applied to the reconstruction design of expressways and first-class highways.