In this paper, the collective dynamic behaviors of N two-level atoms pumped by two-mode light field in a harmonic trap are investigated. When atoms are driven by the intense pump field, the atomic oscillating modes in the trap are continually excited by the photon recoil effect, which discretely changes the atomic detuning from the pumping field. Under proper resonant conditions the weak probe field will be greatly amplified and the atomic spatial structures will be spontaneously formed in the phase space.