Abstract: The LINAC to ring beam transport line (LRBT) of China Spallation Neutron Source (CSNS) connecting the LINAC and the rapid cycling synchrotron (RCS) transports 80 MeV H^- beams for RCS injection. Effects of space charge of the beam in LRBT due to small emittance and high current is a major cause of emittance growth and beam loss and therefore become important issues in the LRBT design. An achromatic transverse optical matching was operated by Trace 3-D code. Emittance growths of beams with different initial distributions in several LRBT lattices were studied separately. Simulation results show that the LRBT design with Triplet can mitigate the emittance growth by adjusting the lattice of the front matching part and no beam loss occurs at 15 mA. The location and parameters of the debuncher in LRBT were also optimized to reduce the momentum spread and energy jitter.