Aiming at the wettability evaluation of fracturing oil displacement agents in low-permeability tight reservoirs，the error and uncertainty of the existing contact angle method were analyzed. An improved method for evaluating the wettability of fracturing oil displacement agents was proposed. The wettability tendency index，i.e.，the ratio of wetting contact angle between oil displacement agents and distilled water on the hydrophilic/lipophilic surfaces，was used to characterize the wettability change direction and degree of fracturing oil displacement agents. The results showed that there was some uncertainty in reflecting both the direction and ability of the oil displacement agent to change the reservoir wettability by the contact angle and the maximum value of the contact angle change. Hydrophilic surfaces were very stable and had small deviations in wetting contact angle，while lipophilic surfaces had large deviations in wetting contact angle. The relative average deviations of the wetting contact angles between distilled water and hydrophilic/lipophilic surfaces were 0.281% and 8.929%，respectively. In addition，the differences in the maximum change values of contact angles of some oil displacement agents on hydrophilic/lipophilic surfaces were not obvious. The wettability change degree of oil displacement agents could be better characterized by wetting tendency index. The uncertainty of wetting tendency index was much smaller than that of contact angle. The uncertainties of contact angle（wetting for 30 s）of 0.2% sulfonate oil displacement agents，polyoxyethylene ether oil displacement agents and fluorocarbon oil displacement agents on hydrophilic surfaces were 1.24，3.11 and 0.64，respectively，while their uncertainties of water wetting tendency index were 2.48×10^-2，6.33×10^-2 and 1.76 × 10^-2，respectively. The water-wetting index of natural rock samples under the effect of seven fracturing oil displacement agents was measured by the Amott method. The order of this water-wetting index from largest to smallest was consistent with the order of water-wetting tendency index from smallest to largest measured by the contact angle method. The test results of both methods further confirmed the accuracy and superiority of wetting tendency index. This method was beneficial for quickly evaluating the ability of fracturing oil displacement agents to change reservoir wettability and for preferentially selecting fracturing oil displacement agents for low-permeability tight reservoirs.