The measurement of optical signal noise ratio (OSNR) degradation of optical network elements is analyzed theoretically and experimentally. The formula denoting the difference of OSNR detected at input port and output port of the element is derived. Theoretical results show that the difference can not reflect the effect of the element on signal in optical domain under general condition, and can not be taken generally as OSNR degradation for evaluation in the element until the element system meets the demands of condition C=1 given in the paper. Experimental results support the theory and show that for an optical network element with OSNR degradation of 4.5 dB at C=1, when C decreases from 3 to 0.16, the difference of OSNR detected at its input port and output port varies from 1.8 dB to 11.3 dB.