Objective To explore the value of dynamic MRI imaging in investigate the morphologic and dynamic factors of low anterior resection syndrome(LARS)after anal preservation surgery in patients with low rectal cancer.Methods Thirty-five patients who developed LARS after anal preservation surgery for low rectal cancer were prospectively collected.Subjects were underwent routine MRI and dynamic MRI of pelvis 1 week before and 3 months after surgery respectively.Routine MRI was used as the resting phase,and dynamic MRI was used to obtain the rapture phase and forceful phase.Parameters such as anal rectus angle(ARA),the length of the line connecting lower edge of pubic symphysis to the posterior wall attachment point of puborectal muscle(H-line),the vertical length from posterior wall attachment point of puborectal muscle to pubococcygeal line(M-line),thickness of puborectal muscle,and thickness of internal and external anal sphincter were measured at the three time phases.The differences between preoperative and postoperative,and moderate and severe LARS patients were compared by measuring relevant indicators.Results The preoperative and postoperative ARA,H-line length and M-line length of LARS patients were the smallest in the rapture phase,the largest in the forceful phase,and the middle in the resting phase.The postoperative thickness of puborectalis muscle and internal and external anal sphincter were the largest in the rapture phase,the smallest in the forceful phase,and the middle in the resting phase.There were significant differences between the three phases(P＜0.05).There were no significant differences between the three phases of postoperative puborectalis muscle thickness and internal and external sphincter thickness(P＞0.05).The postoperative ARA was greater than preoperative ARA in LARS patients,and the thickness of puborectalis muscle,the internal anal sphincter(resting phase and rapture phase),and the external anal sphincter were smaller than preoperative ARA,and the differences were statistically significant(P＜0.05).The ARA in resting phase and rapture phase was greater in patients with severe LARS than that of patients with mild LARS,and the thickness of internal anal sphincter in resting phase was less in patients with severe LARS than that of patients with mild LARS(P＜0.05).However,the differences between the other indicators were not statistically significant(P＞0.05).Conclusion Dynamic MRI examination technology combines dynamic images with quantitative indicators can be used as an important evaluation basis for LARS patients.