An optical detection system, a blood cell analyzer and a platelet detection method are provided. The optical detection system includes: an optical subsystem, a flow chamber and a first detector; the optical subsystem includes a laser, a front optical assembly including an optical isolator, and a rear optical assembly including a blocking diaphragm. The laser is configured to emit a laser beam; the front optical assembly is configured to perform front optical treatment; the rear optical assembly is disposed downstream of the flow chamber in the propagation direction of the laser beam, and is configured to perform rear optical treatment on the scattered light and the laser beam converged at the blocking diaphragm; and the optical isolator is configured to isolate reflected light that is generated when the laser beam passes through the flow chamber.