By numerically integrating the one-dimensional time-dependent Shrodinger equation, the high-order harmonic generation (HHG) spectrum of a model atom irradiated by two-color coherent fields with frequency ω and 1.5ω is calculated respectively. It is found that there exist odd harmonics, even harmonics and semi-integer harmonics due to the coherent interaction between two colors. The origin of these harmonics and main factors influencing the harmonic efficiency are discussed in detail.