A miniature multiplexer based on Film Bulk Acoustic Resonator (FBAR) is designed for Multiple-Input Multiple-Output (MIMO) terminal combined 1900MHz PCS duplexer with 800MHz Cellular duplexer in this paper. FBAR multiplexer is simulated based on a new FBAR model which combines Mason equivalent circuit model with parasitics model. The parasitics, including RF pads, bonding wires and substrate coupling, have some effects on multiplexer insertion loss and stopband rejection. The methods of modifying bandwidth, improving power capacity and temperature coefficient are also discussed.