This study presents a novel algorithm for radar imaging of high-speed manoeuvering target. It is based on the fact that the azimuth part of the echo is modelled as the superposition of multiple quadratic frequency modulated (QFM) signals, and the range-Doppler algorithm is inappropriate to generate a focused radar image in this case. In this study, the improved version of product high-order ambiguity function (IPHAF) is proposed to estimate the parameters of multiple QFM signals. This estimator requires only one-dimensional (1D) maximisations without the scaling operation compared with the traditional product high-order ambiguity function. The radar image quality can be improved by the IPHAF technique, and the experimental results validate the effectiveness of the method presented in this study.