The invention relates to the technical field of up-conversion from baseband signals to radio frequency signals, in particular to a full-digital direct up-conversion circuit. The circuit comprises a baseband processing module and an RF processing module, wherein the baseband processing module carries out inwards-inserting processing on baseband signals input to a system to improve the data rate, then amplitude reduction is carried out, multi-bit parallel data of the signals are modulated to single-bit serial data, and a noise spectrum is moved. The RF processing module carries out N-times multiplexing on output data after processing through the baseband processing module, the output data are converted to parallel data, and then the parallel data are converted to serial data which are output. The circuit is little in noise interference, simple in structure, easy to implement and low in manufacturing cost.