This paper proposes an efficient geometric transformation method in order to improve the computing efficiency, which is low caused by the conventional backward mapping method, in Digital Phosphor Oscilloscope (DPO) image processing. To make use of the feature of DPO image, the proposed method transfers the direction of the backward bilinear interpolation algorithm to forward, so the great calculation relating to background pixels can be ignored. Theoretical simulator and research results show that the proposed method can effectively accelerate the image processing, at the same time, avoid reducing peak signalnoise ratio (PSNR). Its effect is more obvious in DPO image processing, which includes many background pixels. Experimental results show that the probability of abnormal capture is improved because of the shorter acquisition dead time, so demonstrate the proposed method is reasonable and effective.