The tension of high-speed railway contact line can directly reflect the contact line running status. However, the current monitoring methods cannot be used to monitor the tension of the contact line directly and effectively. This paper presents a method that selects the compensation device for observation, achieved by monitoring the ratchet angular displacement and vibration of the balance weight tensioner under the condition without affecting safe operation. Then the real-time tension calculation can be realized through Gauss-Seidel iteration algorithm (GSIA) using the monitoring variables. This paper use the method in the field test, the experimental results show that the results of the proposed monitoring and calculation method are consistent with the real measuring result, which indicates the effectiveness of the monitoring method and the accuracy of the calculation method.