The present invention relates to an alkali-free glass fiber having improved property, and a preparation method thereof, wherein the chemical components of the alkali-free glass fiber comprise, by weight, 56-57.5% of SiO2, 3-4.5% of B2O3, 13.5-15% of Al2O3, 19-20.5% of CaO, 2.8-3.5% of MgO, 0.8-1.1% of R2O=K2O+Na2O, 0.2-0.6% of TiO2, and the balance of Fe2O3+ZnO+ZrO2+Cr2O3, and raw material preparation, melting, drawing, spooling, glue immersing, drying and detection are performed to obtain the alkali-free glass fiber. According to the present invention, by adjusting the component ratio of the formula, the pollution caused by boron volatilization is reduced, the environmental protection is easily achieved, the resources are saved, and the powder production cost is substantially reduced; and through the synergistic effect of various components and the strict control of the component content, the prepared alkali-free glass fiber has excellent performance.