An embedded microstructure fiber device with large-core plastic cladding fibers inserted is presented. The refractive index profile of the taper end is uniform, and so is the power distribution. On the basis of the theoretical analysis, the coupling coefficient of the nearest-neighboring fibers is exponentially decreased with the decreasing diameter of the taper. The transmission loss of the device is lower when the length of the taper is longer. This kind of devices are fabricated by the fixed flame method and the movable flame method, respectively. The taper is elongated nearly five times by the movable large-hot-zone fiber-tapering system. The transmission loss of the two devices from one inserted fiber to the taper end is measured with He-Ne laser at about 632.8 nm wavelength. The loss values are about 2.63 dB with the taper length of 0.7 cm and 1.06 dB with the taper length of 3.4 cm. The influence of the taper length agrees with the theoretical analysis, and low-loss device can be achieved by improving the structure.