In order to solve the problem that laser feedback has lock-in zone when measuring the phase delay of small birefrin- gent elements, the feedback effect of combined 1/4 wave plates in the feedback eXternal cavity of He-Ne laser is studied.By placing two 1/4 wave plates in the feedback eXternal cavity,rotating one of the wave plates, and changing the included angle between the fast and slow aXes of the two wave plates, polarization jump will occur.It is found that when the angle between the fast and slow aXes of the two wave plates changes, the polarization jump phenomenon always eXists and the position of the jump point changes, that is, the phase delay of the combined wave plate can change continuously.When the included angle be- tween the fast and slow aXes of the two wave plates varies from 0。to90。, the eQuivalent phase delay of the combined wave plate can be changed from 19.79。to 160.06。through five repeated eXperiments, and the maXimum difference of repeated measure- ments is 0.6。, and the maXimum standard deviation is 0.25。.The combined wave plate can be applied to the measurement of micro birefringent elements in laser feedback measurement.As a bias element,it modulates the phase delay of the measurement system that needs to be compensated.