The aim of this research is to investigate the influence of sunlight on concrete box girder at precasting yard in arid and cold regions with big temperature difference. First of all the materials were tested, and concrete anti-crack ring was used to test the advantages and shortcomings of concrete in curing conditions both in the presence and in the absence of sunlight. Moreover, the variations and losses of concrete strength were also analyzed under the above mentioned curing conditions, and the obtained results were compared with those under standard curing. In addition, the changes of temperature field of precast box girders in the precast yard were tested and the temperature gradients in different time periods were acquired. The results are concluded as follows 1) sunlight enhances the strength of concrete, but exerts adverse impact on the crack resistance of concrete; 2) the losses of tensile strength and compressive strength of concrete under sunlight curing are both larger than those in the absence of sunlight; 3) the temperature variation law of box girder in the precasting yard and the change law of temperature gradient in the same time of a day are given.