The calculation of spatial relative position between the catheter and the vessel centerline was investigated,which is a key in the fusion of the single-plane coronary angiograms and the intravascular ultrasound images for segments of coronary arteries. Firstly, the parameters of geometric transformation matrix were optimized by constructing six pairs of matched points and energy functions. Then, the function relationship among the matched points was figured out based on the method of optimal vertical plane, and the measurement method and error analysis method of the lengths and angles for the spatial relative position between the catheter and the vessel centerline were given. Finally,the relative positions were calculated through the single-plane coronary angiograms. The experimental results indicate that the average error and the standard deviation of reconstruction have reduced to 2.284 3 mm and 2.483 1 mm, the average errors of the length and angle between the catheter and the vessel centerline to 0.498 0 mm and 6.029 9°,and the standard deviation to 0.574 7 mm and 7.861 9°, respectively.It concludes that the calculation of spatial relative position can improve the accuracy of image fusion and can provide an assistance for doctors in the clinical diagnosis of coronary disease and the interventional treatment.