The invention relates to high magnesium content boron-free glass fiber, which has high tensile strength and high elastic modulus, and is boron-free, fluorine-free and alkali-free glass fiber. The composition of the glass fiber adopts a four-element system of SiO2, Al2O3, CaO and MgO as the main. The glass fiber is characterized in that, based on the composition of the SiO2, the Al2O3, the CaO and the MgO, the CaO is substantially replaced by the MgO, and concurrently TiO2, Fe2O3 and other components are introduced and/or added, wherein the mass of the four main components of the SiO2, the Al2O3, the CaO and the MgO is more than 96.5% of the mass of the glass fiber, and the sum of the masses of the remaining secondary components including the TiO2, the Na2O, the K2O, the Fe2O3 and other inevitable impurities is 1.5-3.5% of the mass of the glass fiber. According to the present invention, the tensile strength and the elastic modulus of the glass fiber of the present invention are significantly higher than the tensile strength and the elastic modulus of the ordinary alkali-free glass fiber, and the glass fiber of the present invention has excellent acid resistance. According to the glass, high melting temperature and high fiber forming temperature are required, wherein the difference value between the fiber forming temperature and the liquid phase line temperature is more than 70 DEG C, and the condition of the industry production of the glass fiber is met.