An oscillator with the function of frequency extension was proposed to suppress the electromagnetic interference(EMI) problem in LED driver. With the clock frequency modulated by frequency extension technology, EMI can be restrained effectively, and two working modes of frequency-extension mode(FEM)and fixed frequency mode(FFM)can be choosen, which drifts little across temperature. Process simulations based on 0.18μm BCD show that the clock was extended 6.09% up and down the central frequency of 558kHz in a period of 57.8μs, and EMI power reduces by 12dB. Due to its inhibition of EMI, the circuit was suitable for LED driver power supply that requires higher electromagnetic compatibility.