Objective: The purpose of this study was to explore the optimal time of laparoscopic cystectomy for unilateral ovarian endometrioma patients and to evaluate the influence on the ovarian reserve. Design: Prospective randomized controlled study. Setting: University hospital Population or Sample：88 patients received their first identified diagnosis of ovarian endometrioma by ultrasound (>4 cm) and were given oral contraceptive pills (OCP, drospirenone and ethinylestradiol) for one cycle before laparoscopy. Methods：Randomly divided into two groups: laparoscopy at Late luteal phase (group LLP) (n=44): Termination of OCP for 2 days; and laparoscopy at Early follicular phase (group EFP) (n=44): Day 3 after menstruation. Main Outcome Measure(s): Serum Anti-Müllerian hormone (AMH) and Leukocyte esterase (LE) levels of endometrioma wall were measured. Assessment of ovarian reserve damage based on alterations in the serum AMH levels after unilateral ovarian endometrioma surgery. Result(s): Preoperative serum AMH levels of both group decreased from preoperative to post-operative 1 week and post-operative 6 months, while difference values of group EFP were larger than those of group LLP at post-operative 1 week and post-operative 6 months respectively(1.87±0.97 vs 1.31±0.93, P＜0.01; 1.91±1.06 vs 1.54±0.93, P＜0.01)；the mean rates of post-operative serum AMH decline were 37.92% and 46.34% in group EFP respectively, which were significantly higher than those of group LLP (25.83% vs 31.43%，P＜0.01). Ovarian endometrioma wall AMH of group LLP was significantly lower than that of group EFP ([22.86±3.74] vs [31.02±5.23], P＜0.01)；While ovarian endometrioma LE concentration of group LLP was significantly higher than that group EFP([482.83±115.88] vs [371.68±84.49], P＜0.01). And significant negative correlation between leukocyte esterase and AMH concentration in the cyst wall of ovarian endometrioma (r=-0.564，P＜0.01). Conclusion(s): The optimal time for laparoscopic cystectomy for patients with first identified unilateral ovarian endometrioma is late luteal phase, which reduces ovarian tissue loss and preserves ovarian reserve effectively and safely.