[en] Beams emittance is a very important parameter of the RF-injector, it is growth due to RF field space charge effects and so on. The analytical expression of beams emittance in RF injector has been given in this paper, it shows that beams emittance is various in RF injector. The dependences of beams emittance at the exit of a 1 + 1/2 cell photo-cathode injector on field strength, initial phase, beam size, bunch length and charge, etc are researched respectively using SUPERFISH and GPT programs. Beams emittance of less than 2πmm·mrad can be obtained by appropriately choosing these parameters