The open line test must be done before HVDC transmission project is put into operation. The relationship between firing angle and DC voltage at open line test is not same as normal operation. Based on the circuit model of the valve and circuit principle, a detailed analysis about the turning on/off of the valve without connecting the DC line is given. A relationship between firing angle and DC voltage is obtained. And the scope for applying this relationship is given. A simulation in EMTDC software is employed to verify the analyzing results. Analysis and experimental results show that: without DC line, DC voltage will reach its rated value when firing angle is less than 60 degrees; the protection based on the voltage criterion in open line test is not reasonable. Only the current criterion in protection is needed for detecting whether the open line test is successful or not.