The magnetic fluid sensor is proposed and numerically investigated by using finite-element-method (FEM) based on the photonic crystal fiber (PCF) with butterfly-shaped air holes and a ring core. Ethanol is used as a magnetic fluid that is injected in the ring core which provides unique characteristics. The goal of the proposed structure is to gain a better response to birefringence and relative sensitivity. In this work, the observed birefringence is 1.27×10−2$1.27\times 10-2$; the sensitivity responses are 40 nm/Oe, and 94047.13 nm/RIU can be gained for the variations of magnetic field strength from 50 to 200 Oe and 200 Oe, respectively. With best of knowledge, this sensitivity response is the highest response in the magnetic sensing. For better sensing performance the proposed butterfly structure can be used in the bio-optics field and nanofluidic technologies.