Abstract: Flaw sizing is the focus of research in the flied of non-destructive testing. The diffusion of ultrasonic waves will cause edge blur of C-scan images, which affects the accuracy of flaw sizing. Based on the multi-Gaussian beam model, according to the propagation law of the ultrasonic wave at a curved interface, the point spread function (PSF) of the rod water-immersion ultrasonic imaging system is deduced. The nylon rod with flat-bottomed holes is scanned by a four-axis ultrasonic water-immersion testing system to obtain the C-scan image, and then the C-scan image is deconvoluted with the Richardson-Lucy iterative algorithm. The 6 dB drop technique is used to quantitatively analyze the original image and the deconvoluted image. The results indicate that the proposed deconvolution method exhibits a higher sizing accuracy. In particular, the error of the 1.5 mm diameter flat-bottom hole is reduced from 88% to 18.7%.