Background: The diagnosis of cancer can motivate patients to change their physical activity habits. No data has reported level of physical activity before and after breast cancer diagnosis in Chinese women. Methods: In an on-going prospective cohort study which involved 1462 Chinese women with early-stage breast cancer, a validated modified Chinese Baecke questionnaire was used to assess physical activity at baseline, 18-, 36- and 60-month after diagnosis. At baseline, patients recalled their habitual physical activity in the preceding 12 months before cancer diagnosis. At 18-, 36- and 60-month follow-up, patients reported their habitual physical activity over the previous 12 months. The level of physical activity at post-diagnosis was defined as the average value assessed at 18-, 36- and 60-month follow-up. Results: Breast cancer patients significantly increased level of physical activity, with median value of 0.6, 5.3, 4.4 and 3.9 MET-hours/week at baseline, 18-, 36- and 60-month follow-up. The average level of physical activity at post-diagnosis was also significantly higher than that at pre-diagnosis (P< 0.001), with median value of 5.8 MET-hours/week. However, there was no significant difference between any two follow-ups at post-diagnosis. The proportions of participant who met the exercise recommendation (according to WCRF/AICR, 10 MET-hours/week) were low at pre- and post-diagnosis, being 20.7% and 35.1%, respectively. Compared to pre-diagnosis, most of the patients improved or had no change on level of recreational physical activity at post-diagnosis, with the respective proportion being 48.2% and 43.8%. Multivariate analysis showed that higher increase in physical activity after cancer diagnosis was observed among breast cancer patients who were married or cohabitation, unemployed (compared to full time) and had no comorbidity (compared to patients who had one comorbidity). Conclusions: Chinese breast cancer patients reported significant and long-term changes in physical activity after cancer diagnosis, which was in line with current recommendation. However, the proportion of patients who met the exercise recommendation for cancer survivors was still low. Empowering patients on the importance of durable high level of physical activity in breast cancer survivorship is warranted.