The horizontal monitoring force methods were discussed from the strain, displacement response and motion attitude  based on an on-site soft yoke mooring system of FPSO in service in Bohai Bay. The precision and accuracy of the three methods were also analyzed. The results show that the three methods have approximately the same precision, and the motionbased inclinometers are more stable and inexpensive than the others. In general, the present work can not only ensure the safty of  the mooring system of FPSO, but also provide important reference for performance evaluation of soft yoke mooring system.