The stayed cables of cable-stayed bridges are the main load-bearing components. The cable tension and its variation are two of the most important criteria for judging if a cable-stayed bridge is in order. Through monitoring the cable tension and its variation, the overall technical condition of a cable-stayed bridge can be estimated and whether the cable socket system and protecting system are functional and whether the steel wires in the stayed cables are rusted can be checked. In this paper the frequency analysis method for in situ measuring of cable tension with polyvinylidene fluoride (PVDF) piezoelectric films is put forward. The relationship between the cable sag and the cable tension is explored and the disadvantages of the traditional measuring method of cable tension with frequency analysis are also represented, as are the explicit error equation and its compensation methods. The simultaneous experimental results on a stayed-cable model and the comparison results measured with the accelerometer are presented.