In this study，we designed a Sub-6GHz multi-mode multi-band transmitter using a standard 65 nm CMOS process to improve the CIM3. The transmitter comprises a bandwidth-reconfigurable sixth-order low-pass filter，programmable gain amplifier，I/Q quadrature mixer，and driver amplifier. To improve CIM3，we applied transconductance boosting techniques in the programmable gain amplifier to reduce the third-order harmonics of the analog baseband expected signal. Additionally，we utilized an LC resonant cavity in the I/Q quadrature mixer to filter out the HD3 at its output. We applied the multiple gated transistor technology in the driver amplifier to mitigate third-order non-linearity and improve linearity. The proposed switchable transformer presents the advantages of high coupling coefficient and low insertion loss. It is used to drive amplifiers and provide the optimal load impedance for low and medium frequency bands. The simulation results indicate that the proposed transmitter achieves a ClM3 of .63，4 dBc，an ACPR of .47. 3 dBc，and an EVM of 1. 66%.