The invention relates to a semiconductor device, a manufacturing method thereof, a test structure of the semiconductor device and a test method. The test structure includes a semiconductor substrate, a floating gate located on the semiconductor substrate, a floating gate oxide located between the semiconductor substrate and the floating gate, a control gate located on the floating gate, a first terminal connected with the semiconductor substrate, a second terminal electrically connected the control gate, and a third terminal electrically connected to the exposed floating gate. According to the test structure and the test method of the invention, an interface layer between the floating gate and the control gate of a surrounding area device can be monitored through online WAT (wafer acceptance test); and when the interface layer exists between the floating gate and the control gate of the surrounding area device, the test structure of the invention can accurately detect the capacitance-current curve of the gate oxide, and the accurate thickness of the gate oxide can be obtained through feedback.