The invention discloses a downlink transmitter of a long term evolution (LTE) base station system. The downlink transmitter includes a base band unit (BBU) and a remote radio unit (RRU), wherein the base band unit (BBU) is connected with the remote radio unit (RRU) through an interface Ir between the remote radio unit (RRU) and the base band unit (BBU), the base band unit (BBU) is used for transmitting original data to the remote radio unit (RRU) through the interface Ir, and the remote radio unit (RRU) comprises an inverse fast Fourier transform (IFFT)/inverse discrete Fourier transform (IDFT) module and is used for receiving the original data transmitted by the base band unit (BBU), and performs inverse fast Fourier transform (IFFT) or inverse discrete Fourier transform (IDFT) on the original data, and thereafter transmits the processed original data. Correspondingly, the invention also discloses a data processing method of the downlink transmitter; and the data processing method of the downlink transmitter can assist in decreasing sampling rate and reducing data bit width, and therefore, data transmission traffic on the Ir can be effectively decreased, and the cost of the transmission from the base band unit (BBU) to the remote radio unit (RRU) can be decreased, and optical fiber resources arranged on the Ir can be decreased, and equipment cost and maintenance cost can be saved.