The invention provides a mask plate and an encapsulating method for an OLED device. According to the mask plate provided by the invention, a plurality of rectangular shielding parts are uniformly arranged on four sides of a rectangular opening of a frame body; when the mask plate is utilized to encapsulate the OLED device, three independent sides of the rectangular shielding parts generate shadow effects, so that film thickness of a passivation layer formed below the rectangular shielding parts is the same as that of a passivation layer formed below a residual un-shielded area in the rectangular opening; moreover, positions on which the rectangular shielding parts are connected with the sides of the rectangular opening do not generate shadow effects, and the shadow effect generated by a bottom side of a first groove between adjacent two rectangular shielding parts is diffused towards the positions on which the adjacent rectangular shielding parts are connected with the sides of the rectangular opening for being diluted and reduced, so that the shadow effects of the side integers of the rectangular opening is reduced, and an actual film-forming area of the passivation layer is almost the same as a preset designed film-forming layer, and therefore, a narrow side frame design can be realized, encapsulating effect is improved, and the service life of the device is prolonged.