Linac injector is a major sub-system of the proton therapy facility. Due to schedule consideration, the Shanghai advance proton therapy facility APTRON uses a linac injector made by ACCSYS in California, USA. Now, we have developed a linac injector made up of Electron Cyclotron Resonance (ECR) ion source and 4-vane Radio Frequency Quadrupole (RFQ), as well as alternating-phase-focused (APF) Drift Tube Linac (DTL), which is a significant technological innovation. During the last 5 years, we have finished a series of R&D stage including physical design, electromagnetic design, mechanical design, assembly and alignment, cavity cold test, RF conditioning and beam commissioning. Finally, we get 7 mA, 7 MeV proton beam from the down stream Faraday cup. According to the measurement of our beam diagnostic and analysis system, the center energy of the beam is 6.975 MeV, the beam intensity within ±0.35% momentum spread is 6.07 mA.