Objective:To evaluate the impact of bladder volume on dosimetric parameters of clinical target volume (CTV) and organs at risk (OAR) of intensity modulated proton therapy (IMPT) for localized prostate cancer during the treatment planning and daily treatment.Methods:Clinical data of 25 patients with localized prostate cancer admitted to Ruijin Hospital affiliated to Shanghai Jiao Tong University School of Medicine from November 2021 to June 2022 and enrolled in the "Proton Therapy System" (SAPT-PS-01) registered clinical trial were retrospectively analyzed. All patients were male and the median age was 72 years old. A total of 30 sets of IMPT plans were obtained. Based on the planning CT (30 sets) and weekly verification CT during treatment (172 sets), bladder volume, CTV and OAR dose parameters were collected. Spearman correlation analysis was used to evaluate the correlation between bladder volume in CT and the dosimetric parameters of CTV and OAR during IMPT plans, and Wilcoxon-Mann-Whitney test was adopted to compare the dosimetric parameters of CTV and OAR among different bladder volume change groups.Results:The V 95% of CTV1 and CTV2 were both 100.0%±0.0% in IMPT plans. Bladder volume was significantly negatively correlated with D mean, V 70 Gy(RBE), V 60 Gy(RBE), V 50 Gy(RBE), V 40 Gy(RBE) of the bladder ( P<0.001, 0.003, <0.001, <0.001,<0.001), and D mean, V 50 Gy(RBE) of the small intestine (both P<0.001). During treatment, bladder D mean, V 70 Gy(RBE), V 60 Gy(RBE), V 50 Gy(RBE), V 40 Gy(RBE)( P<0.001, 0.001, <0.001, <0.001, <0.001), rectal D mean, V 50 Gy(RBE), V 40 Gy(RBE) (all P<0.001), small intestine D mean, V 50 Gy (RBE) (both P<0.001) of patients with bladder volume increase >20% compared to baseline were significantly decreased compared to those in IMPT plans. But CTV1 V 100%, and CTV2 V 95% were significantly decreased too( P=0.029, 0.020). In the bladder volume decreased>20% patients, the D mean, V 70 Gy(RBE), V 60 Gy(RBE), V 50 Gy(RBE), V 40 Gy(RBE) of the bladder were significantly increased compared to those in IMPT plans (all P<0.001). However, a bladder volume reduction of ≤20% and increase of ≤20% from baseline had no significant impact on CTV and OAR dosimetric parameters during treatment. Conclusions:For patients with localized prostate cancer undergoing proton therapy, a certain bladder volume should be ensured during planning CT scans. During the daily treatment, the bladder volume should be maintained between 80%-120% of the baseline level to ensure CTV coverage and good dose sparing to OAR.