Bridge frequencies are important characteristics for its lifecycle condition assessment. The drive-by method, which only requires several sensors installed on the passing vehicle and employs vehicle responses to extract essential information about the bridge, has been a focus in the last two decades due to its low-cost and efficient nature. Such a method typically relies on the vehicle-bridge interaction and assumes that the bridge’s frequency does not change during the vehicle passage. However, practical applications indicate that the bridge frequencies can change during the interaction process, especially when the vehicle mass is comparable to that of the bridge. This paper further investigates the time-varying frequency of the vehicle-bridge interaction system incorporating a two-axle vehicle and a simply supported bridge. Firstly, the semi-analytical solution for time-varying bridge frequency was developed and numerical simulations were performed to verify the solution. Different frequency and mass ratios between the vehicle and bridge were explored. Then, the approach to acquire clear time-frequency representations of the vehicle responses for bridge frequency extraction was presented, and ridge extraction was employed to extract the traces of time-varying bridge frequencies. Finally, laboratory experiments, including a scaled two-axle vehicle and a steel beam model, were conducted and the effectiveness of the proposed method for extracting time-varying bridge frequencies was demonstrated.