A 40 Gb/s optical transmission based on the NSFCNet 400 km×10 Gb/s system is realized without any bit error rate (BER) floor. It is shown that typical 10 Gb/s systems of middle or short distance can be upgraded to 40 Gb/s directly without change the transmission links. However, the chromatic dispersion must be compensated precisely when the system upgrades, because the dispersion tolerance of 40 Gb/s systems is much lower than that of 10 Gb/s system. Meanwhile, the influence of the optical power injected into the fiber links is also investigated. The results show that the effect of the fiber nonlinearity must be considered as well as the improvement of optical signal to noise ratio (OSNR) due to the enlarged signal bandwidth when the bit rate of transmission system is upgraded from 10 Gb/s to 40 Gb/s.