As an important measuring method in velocity measuring field, Particle Image Velocimetry(PIV), which follows the principle of dividing the maximum displacement of tracer particles by the corresponding time, is applied more and more widely in various subjects, and the accuracy of which is influenced by the choice of the time delay to some extent. The existing PIV system usually chooses a fixed time delay, which could not meet the need of the application in measuring the vector of time varying flow field with a relatively high measuring accuracy. Considering the weakness of this, we introduce a new kind of adjustable frame-straddling image formation system for PIV application to improve the accuracy in this paper. The image formation system is implemented mainly because of two parts: a dual CCD camera system which is carefully designed to capture the frame-straddling image pairs of the flow field with an adjustable time delay controlled by the externally trigger signals, and an effective subpixel image registration algorithm, which is used to calculate vector of the time varying flow field on the hardware platform, which generates the two channels of trigger signals with the adjustable time delay according to the instantaneous calculating vector of flow field. Experiments were performed for several time varying flows to verify the effectiveness of the image formation system and the results shows that the accuracy was improved in calculating the vector of the flow field based on such image formation system to some extent.