Related to is a gate on array. A circuit for testing a gate line of an array substrate includes: a test pad and a first switch unit which connects the test pad and the gate line and has an end connected to a control terminal. A voltage is applied to the control terminal to control activation and deactivation of the first switch unit. The gate line is in normal operation when the first switch unit is deactivated, and the test pad tests a signal of the gate line when the first switch unit is activated. In normal display, the first switch unit is deactivated, and the gate line is in normal operation. This can avoid influences of an additional load, which would otherwise cause abnormal display of a picture. In a manufacturing procedure, explosive wound caused by electrostatic discharge of the test pad can be prevented. When a display device cannot be lit, an external voltage can be introduced to activate the first switch unit, so as to detect a signal of the gate line.