A frequency synthesizer and a method for frequency synthesis are provided. The frequency synthesizer includes a subband phase-locked loop (PLL) and a main PLL. A divisor of an adjustable non-integer frequency divider in the subband PLL is adjusted, and a channel frequency of a maximum frequency band of all frequency bands is output. A divisor of an adjustable integer frequency divider in the main PLL is adjusted, and a center frequency required for each frequency band of the all frequency bands is output. The center frequency and the channel frequency are mixed by a mixer to produce a band frequency. A quantization noise generated by the adjustable non-integer frequency divider is filtered out by the subband PLL to reduce the impact of the quantization noise on the main PLL.