By solving the 1 dimensional time dependent Schrodinger equation, the effects of laser frequency and two colour coherence fields on high order harmonic generation (HOHG) of atom under the same laser intensity are studied. The results show that with higher laser frequency, one can get lower order harmonics but higher conversion efficiency; with lower laser frequency, get higher order harmonics but lower conversion efficiency; with two colour laser fields, both the harmonic order and conversion efficiency are raised obviously. With the first harmonic and its second harmonic the even order harmonics can be obtained.