The effect of styrene acrylic (SA) copolymer on the early hydration of calcium sulphoaluminate-anhydrite composite was investigated by calorimetry, X-ray diffraction, thermogravimetic analysis, scanning electron microscopy, and inductively coupled plasma-optical emission spectroscopy. The results show that SA can delay the hydration process, reduce the content of ettringite (AFt), single sulfur calcium sulphoaluminate hydrate (AFm) and Al(OH)3 (AH3) in the accelerated hydration period, and promote the reaction of AH3 with anhydrite and calcium hydroxide to form AFt as well as the transformation of AFt to AFm in the late hydration period. Also, SA can rapidly reduce the surface tension of the pore solution, and the surface tension tends to be stable within the hydration reaction. SA can increase the pH value of the pore solution, increase the concentration of OH- and affect the concentration of Ca2+, SO42- and ［Al(OH)4］- in the composite, and decrease the ion concentration and the precipitation rate of hydration products at the initial stage of hydration, thus delaying the hydration process and reducing the hydration products.