The effects of different length and content of ramie fibers on the water resistance and mechanical properties of ramie fiber reinforced calcined phosphogypsum composite materials were studied through the tests of water absorption, softening coefficient, flexural strength and compressive strength, combined with Fourier transform infrared spectroscopy and scanning electron microscopy. The results show that adding ramie fiber can improve the water resistance and mechanical properties of ramie fiber reinforced calcined phosphogypsum composite materials, and also promote the ductility of composite materials. Adding 0.5% (volume fraction, the same below) 10 mm ramie fiber makes the softening coefficient of composite materials reach the maximum, which increases by 20.0% compared with the control sample. The incorporation of ramie fiber can effectively improve the flexural strength of composite materials. At the age of 28 d, the flexural strength of composite materials with 1.5% 10 mm ramie fiber increases by 39.5% compared with the control sample. The compressive strength of composite materials with less than 20 mm ramie fiber decreases, but increases by addition of less than 1.5% 30 mm ramie fiber. At the age of 28 d, the compressive strength of composite materials with 1.5% 30 mm ramie fiber increases by 10.1% compared with the control sample. Ramie fiber hydrolyzes in the composite materials matrix. The degree of hydrolysis increases and the surface becomes rougher with the increase of age.