The invention discloses a bypass casing for an engine and belongs to the field of design of engine casings. The bypass casing comprises an outer casing, an inner casing, junction boxes and a support; a core engine unit body (6) of the engine is sleeved with the inner casing (2); the inner casing (2) is sleeved with the outer casing (1); a flow channel is formed between the outer casing (1) and the inner casing (2); multiple junction boxes are arranged in the flow channel between the outer casing (1) and the inner casing (2) so as to lead out pipelines; the support and the core engine unit body are separately mounted through bolts; and the other end of the support adopts the arc-shaped structure and is glidingly connected with the back section of the inner casing. The outer casing forms an upper wall surface of the bypass flow channel, the inner casing replaces the core engine unit body to form the lower wall surface of the bypass flow channel, the flow channel formed by the outer casing and the inner casing effectively reduces the total pressure loss of the bypass, the total pressure recovery coefficient is increased, the vibration effects caused by air flows on the engine structure and accessories are reduced, and the structural reliability is improved.