A new on-line monitoring scheme of oil-immersed current transformer based on micro-electro-mechanical systems (MEMS） optical fiber sensing technology is proposed and studied in view of the shortcomings of gas chromatography, such as the need to take oil samples regularly and the discontinuity of monitoring. This paper introduces a MEMS fiber optic sensor which integrates temperature-pressure and hydrogen-acetylene detection modules. Through finite element simulation, an electromagnetic model of the oil-immersed current transformer is developed. The feasibility of scheme is demonstrated by comparing pre and post-sensor installation electromagnetic distributions and secondary current waveforms. Heat-flow coupling simulation analyzes insulation oil temperature distribution and flow characteristics, highlighting effective sensor monitoring of faulty gas at the oil inlet. The built MEMS optical fiber online monitoring system for 110kV three-phase oil-immersed current transformers is validated through field experiments, enabling real-time acquisition of oil temperature, pressure, hydrogen, and acetylene parameters. Successful detection of excessive dissolved hydrogen content in B-phase oil underscores its potential in early fault monitoring and prevention.