【Abstract】 Objective When knee medial collateral ligament (MCL) rupture, the upper surface of medial meniscus is exposed totally, like the gulf panoramic, which is called “panoramic views of the bay sign” or the “bay sign”. To investigate the reliability and significance of the “bay sign” in diagnosis of knee MCL rupture under arthroscope. Methods Between March 2007 and March 2011, 127 patients with knees injuries were divided into the observation group (n=59) and control group (n=68) based on the MRI results. In the observation group, 59 patients had MCL rupture by MRI, including 12 cases of MCL injury alone, 16 cases of MCL injury with lateral meniscus torn, 27 cases of MCL injury with anterior cruciate ligament (ACL) injury, 3 cases of MCL injury with ACL and posterior cruciate ligament (PCL) injury, and 1 case of MCL injury with patellar dislocation; there were 38 males and 21 females with an average age of 23.2 years (range, 16-39 years). In the control group, 68 patients had no MCL rupture by MRI, including 38 cases of ACL injury, 4 cases of ACL and PCL injury, and 26 cases of ACL and lateral meniscus injury; there were 45 males and 23 females with an average age of 31.8 years (range, 25-49 years). The “bay sign” was observed under arthroscope in 2 groups before and after operation. Results The positive “bay sign” was seen under arthroscope in the patients of the observation group before MCL repair; the “bay sign” disappeared after repair. No “bay sign” was seen in patients of the control group before and after ACL reconstruction. Conclusion The “bay sign” is a reliable diagnostic evidence of MCL injury. It can be used as a basis to judge the success of MCL reconstruction during operation.