This paper propose a new type sagnac magnetic field fiber sensor based on the magnetorefractive effect of magnetic fluid.In the single mode-no-core- single mode (S-N-S) structure,when the external reflective index is less than but close to that of no-core fiber,the fiber can be seen as a step-index multimode fiber (MMF).The tran smission spectra of the sensor under different external magnetic fields are mea sured,and is the magnetic field sensing performance studied.The results show that due to the magnetic field can change the direction of ligh t propagation,with the magnetic field strength perpendicular to the sensor direction increasing,the magnetic-fluid-c lad′s reflective index decreases,and the interference peak intensity of the transmission spectrum increases.The w hole experimental device is placed under 20℃,the sensitivi ty of the magnetic field sensing is at 1580.1nm is 0.33dB/m T,and the sensitivity at 1595.1nm is 0.32dB/mT.Compared wit h the previous,the proposed sensor has higher sensitivity,a stable extincion and no hysteresis phenomenon.The corresponding photoel ectric system is built to convert the detection of variation in light intensity into the voltage measurement,the relat ive error is less than 3%,and it has good application prospect.