The modes competition characteristics in birefringence cavity laser are studied in different regions of the gain curve. The mode's intensity modulation depth with modes competition is much deeper than that without modes competition. When the average intensities of the two modes are comparable, the intensity modulation depth of either mode reaches its maximum. Modes competition can do more contribution to the mode's modulation depth than the percentage of light reflected back into the laser cavity does. These characteristics can be used to improve the sensitivity of an optical feedback system. A modes competition factor is introduced to either mode's intensity expression which describes the laser intensity more precisely.