A 40×40 Gbit/s multi-wavelength carrier-suppressed return-to-zero (CS-RZ) format optical transmitter is designed. Based on this transmitter and the experiment of one channel 400 km×40 Gbit/s optical transmission which was upgraded from the National Natural Science Foundation of China Net (NSFCNet, 400 km×10 Gbit/s), a 40×40 Gbit/s wavelength division multiplexing (WDM) optical transmission is also realized. The results show that it is feasible to upgrade a middle and short haul 10 Gbit/s WDM optical transmission system to 40 Gbit/s. However, the chromatic dispersion must be compensated precisely in every channel, since the dispersion tolerance of 40 Gbit/s system is lower than 60 ps/nm and the dispersion slope of the transmission optical fiber does not match well the dispersion slope of the dispersion compensation module. The results also show that the system optimization or the links renewal is necessary for 40 Gbit/s WDM system.