The invention discloses a dual frequency multiplexer by which a first and second coaxial harmonic oscillator type band pass filters are disposed in a box. The box includes a base body, a cover plate and a cover body. The two coaxial harmonic oscillator type hand pass filters are located on the base body and spaced each other by a metal plate; the multiplexer port, first and second ports are positioned on lateral side of the base body. The blocking capacitors are contained in the coaxial chamber of the two coaxial harmonic oscillator type band pass filters. The cover plate is secured on the base body; the first and second direct current circuits are placed on the cover plate; the low pass filters of the first and second direct current circuits are fixed on an edge of a top surface of the coaxial chamber by means of a support member; the cover body and the base body are fastened with each other. The blocking capacitors each are of distributed parameter capacitor. Utilization of distributed blocking capacitors makes the product of the invention small. Moreover, improvement of the structure of the invention brings effect such as less differential loss, large power capacity, as well as high isolation degree between circuits.