Abstract: When utilizing the opportunity illuminator to implement target detection, target signal is often submerged in the strong direct signal and multipath clutter. At the same time, the masking effect of strong echoes seriously affects the weak target detection. Considering the above problems, we proposed a clutter cancellation method based on variable forgetting factor recursive least squares algorithm and an improved CLEAN algorithm based on interpolation to suppresses the strong echoes. Firstly, we make the two-dimensional correlation function with the reference signal and the surveillance signal after clutter cancellation. Then the time delay and Doppler frequency can be estimated from the ambiguity function. Finally, we can suppress the strong target utilizing the interference matrix and thus the weak targets are able to be detected. The simulations proved that the proposed method was insensitive to the time delay and Doppler of the strong echo, and it achieved good detection performance when the arrival time of strong echoes was fractional sampling interval.