The lift-off effect is a main challenge for the eddy current testing (ECT)-based rail detections. The non-coaxial transmitter-receiver (TR) probes are considered as promising structures, however, the research focused on the transmitter-receiver coil distance optimization is limited. In this study, this coil distance is optimized for the Tx-Rx probe with varying lift-offs in rail inspections. Firstly, the solution for the analytical model of the Tx-Rx probe is given. Based on the model, lo–D integration is introduced, and an optimization method is given. Analytical simulations under different conditions show that the optimized coil distance can reduce the lift-off effect, and is only related to the maximum value of lift-off and the out radius of the coils. This approach can also be applied to specimens with different materials and thicknesses. The proposed method provides an optimization method for coil parameter design.