This paper analyses the formation mechanism in NH3 multiphoton ionization mass spectra induced by an XeCl excimer laser. Established model includes that NH3+ are mainly yielded in (2+1) multiphoton. ionization via the Eydberg states B(6) and C (1), and NE4+ formed from ion-molecule reaction, NH3+(NH3, NH2)NH4+ and very low-yield NH2+ most probably produced from (2 + 2) multiphoton ionization via resonant intermediate state B(6). The rate equation solution for this model is in good greement with our experimental results. It predicts that the information oil direct or Indirect ionization mechanisms are involed in the dependences of ion intensity on laser pulse duration.