Objective To explore the optimal scanning phase of multi-slice spiral CT dynamic enhanced scanning in the diagnosis of Pulmonary Embolism (PE) and optimize the imaging diagnostic protocol. Methods: 102 patients with suspected pulmonary embolism who visited our hospital from July 2023 to October 2024 were selected as the research subjects. All patients underwent dynamic enhanced scanning with multi-slice spiral CT, and images of the pulmonary artery phase (T1) and three subsequent phases (T2, T3, T4) at 5-second intervals were obtained respectively. Based on whether 102 suspected patients with pulmonary embolism developed pulmonary embolism, the patients were divided into the PE group (with pulmonary embolism) and the non-PE group (without pulmonary embolism). Analyze and compare the CT values, image quality and diagnostic efficacy at different time phases, and analyze the sensitivity, specificity and accuracy. Result: Among the 102 suspected PE patients, 58 patients developed pulmonary embolism and were included in the PE group. There were 44 patients without pulmonary embolism and they were included in the non-PE group. There were no significant differences in age, gender and BMI between the PE group and the non-PE group (P>0.05). The D-dimer level in the PE group was significantly higher than that in the non-PE group (P<0.05). There were significant differences in CT values between the PE group and the non-PE group at each time phase (P<0.05). The CT values of the lesion segments in the PE group were significantly lower than those of the normal blood vessels at the same level in the non-PE group (P<0.05). The CT values of the pulmonary artery in stage T2 showed the most significant differences among groups (P<0.001), and the sensitivity (94.83%), specificity (93.18%), and accuracy (94.12%) were all significantly higher than those in other phases. The image quality score was the highest, the noise was the lowest, and the signal-to-noise ratio was the best in stage T2 (P<0.05). Conclusion: Stage T2 is the optimal time phase for dynamic contrast-enhanced scanning in the diagnosis of PE, which can significantly improve the diagnostic efficiency and image quality.