A kind of magnetic-fluid-cladded photonic crystal fiber coupler is designed and investigated experimentally. The coupling ratio of the coupler can be adjusted online by magnetic field. Experimental results indicate that the smaller the coupling area size of the as-fabricated coupler is, the higher the sensitivity of the coupler with respect to magnetic field is. The coupling ratio of the as-fabricated coupler with coupling area size of 27 μm changes linearly with the external magnetic field within the range of 0-14 mT. The achieved sensitivity is 0.0289 mT-1. But the influence of ambient temperature on the coupling ratio will increase when the coupling area size of the as-fabricated coupler decreases. For practical applications, the trade-off between the magnetic tunability of the coupling ratio and the temperature response should be considered comprehensively and the photonic crystal fiber coupler with appropriate parameters should be chosen.