Output signal efficiency of low voltage power line carrier(PLC) is a very important part for successful communications. This paper discusses the impact on output signal of PLC based on test data of different electricity load running on the low voltage power grid. The test method is stabilizing output impedance of low voltage PLC, then replacing the different electricity load and capturing the output signal of PLC through spectrum analysis device finally. The impact degree is evaluated on output signal of low voltage PLC by different electricity load. This test can realize the actual running condition of low voltage PLC, reflect output ability of PLC restricted by electricity load and improve performance of PLC production.