It is a challenging problem for the detection of high-speed target with small reflectivity. A serious range walk will result from high-speed targets; the low pulse repetition frequency will also lead to cross-range undersampling. Failure to the conventional Keystone transform will not contribute to the effective accumulation of target energy. And the different range walk ratio will affect multi-target detection. A method of the detection for weak targets based on Clean technique is presented to correct the range walk and the quadratic phase term, thereby achieving the effective accumulation of target energy. Clean technique is also used to improve the detection of weak targets.