Various photodissociation pathways of methylamine involving the three lowest electronic states, namely, singlet ground S-0 state, singlet first excited S-1 state, and triplet ground T-1 state, were studied by the (MS-)CAS(8e,8o)PT2/6-31++G** method. All critical points, i.e., minima, transition states, minimum energy conical intersections, and minima on the seam of crossing, were explored systematically by the global reaction route mapping (GRRM) strategy utilizing the anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods. On the basis of obtained structures, we discuss the photodissociation mechanism of methylamine in the experimental excitation wavelength range 222-240 nm in detail. Especially, the T-1 potential energy surface was explored systematically for the first time. The N-H bond rupture is a primary channel on the S-1 state. Along the N-H dissociation path on S-1, there is a low-energy conical intersection (CI), and through this CI the system can go back to the S-0 state; from the CI the system can directly dissociate to CH3NH + H or reproduce the original CH3NH2 on S-0. There is a seam of crossing between S-0 and T-1 in a partially dissociated CH3-NH2 geometry, and through this seam the system may go up to the T-1. On the T-1 state, a roaming-like pathway giving CH4 + NH (X-3 Sigma(-)) products was found, which would explain the recently proposed intersystem crossing mediated roaming dynamics.