The launch of the unmanned craft took place from the Wenchang base on Hainan Island.
The second module consists of three sleeping places and a space for scientific experiments. The module is almost 18 meters long and weighs 22 tonnes. The first module, Tianhe, was postponed last year.
The new module will dock with Tianhe, which according to experts will be a complicated operation that requires high precision.
The third and final module is scheduled to be launched in October and the space station is expected to be completed by the end of the year. Its lifespan should be at least 10 years.
Dictatorship China is not part of the collaboration that led to the International Space Station (ISS), but is keeping a high pace in its own space program.