The influences of correlative parameters in the process of fabricating fused biconical taper fiber wavelength division multiplexer on the minimum channel spacing are studied, and an improved device for fused biconical taper is designed according to the conclusion obtained by numerical simulation, which has an electric strip heater with long uniform heating area as the heating unit. The coupling length is lengthened obviously by using the improved device, and the limitation of conventional drawing method to the reachable minimum channel spacing of fused biconical taper fiber wavelength division multiplexer is broken. A preliminary experiment result shows that the minimum channel spacing reaches 3.01 nm.