A handwriting data processing method is applied to a pen display having wireless communication function and a data processing device. The handwriting data processing method includes the steps of: the data processing device obtaining a handwriting data from the pen display in a wireless communication manner; the data processing device generating a compressed screen image and transmitting the data of the compressed screen image and the handwriting data, which is not compressed, to the pen display in the wireless communication manner; the pen display uncompressing the data of the compressed screen image and overlapping the uncompressed screen image and the handwriting data to form a complete screen image and displaying the complete screen image. By the handwriting processing method, the machine time of the processor of the pen display is effectively lowered, significantly reducing the delay phenomenon of the displayed handwriting.