The adjustable coupling effect of plastic fiber dual taper and semisphere medium cavity implemented by using He Ne laser is reported. The emitting efficiency of fiber dual taper is measured. The coupling efficiency of semisphere cavity relative to surface area of semisphere and distance between dual taper and semisphere cavity is also measured. Based on to this effect, an adjustable plastic optical fiber coupler can be made. Furthermore, add drop devices suitable to plastic optical fiber transmission networks can be constructed by adding a tunable filter.