In this paper,the phase properties of two-mode SU(2) coherent field interacting with A-type three-level atom are investigated. We find that single-mode phase and the sum of the two-mode phase are all randomly distributed. The expressions of average value and variance of the phase difference are presented. The effects of atomic initial state and the atom-field coupling strengths on the variance of the phase difference are examined. That the atomic coherent trapping will be taken place under an appropriate condition is verified.