Abstract: GNSS time comparison is currently the most widely used time comparison method. Although the performance of GNSS single-difference carrier phase time comparison technology deteriorates with the increase of baseline length, it still has obvious advantages in accuracy and convergence time under short baseline conditions. Traditional GNSS single-difference carrier phase time comparison technology requires the use of dual-frequency observations to complete the fixing of single-difference ambiguity. In order to achieve high-precision time comparison using only single-frequency observations, a novel GNSS single-difference carrier phase time comparison method with double-difference integer ambiguity constraints is proposed in this paper. This method reduces the dimension of the parameters to be estimated by constraining the double-difference integer ambiguity, thereby improving the estimation accuracy of the single-difference ambiguity. The results show that the time comparison accuracy of the proposed method can reach 0.03 ns, and can converge to within 0.2 ns in about 100 s, making it particularly suitable for applications requiring high-precision and fast time synchronization at short baseline.