Embodiments of the present application relate to the field of fiber communications, and provide an optical add drop multiplexer and an optical signal transmission method. The optical add drop multiplexer comprises N cascaded filter units, N being an integer greater than 1. Each filter unit comprises a primary microring group, a secondary microring group, and a phase bias device. The primary microring group comprises: at least one first microring and a first TC. The second microring group comprises at least one second microring, the radius of the second microring being different from that of the first microring. A downloading port of the primary microring group is connected to an input port of the secondary microring group. A through port of the primary microring group is connected to the phase bias device. When wavelength switching is needed to be performed on an optical signal downloaded from a channel, by further controlling the channel to be in an optional signal downloading state by means of the control of the first TC after the switching is completed, the present application effectively reduces the loss of optical signal transmission in an optical add drop multiplexer.