Objective: Develop a reliable single-shot ${\rm{T}}_{{2}}$ mapping method with extra robustness to motion and the potential for real-time dynamic and quantitative MR imaging. Methods: A single-shot ${\rm{T}}_{{2}}$ mapping sequence was proposed based on spin-echo planar imaging acquisition scheme. Two overlapped echo signals with different ${\rm{T}}_{{2}}$ weighting were obtained simultaneously by using two small flip-angle excitation pulses and corresponding echo-shifting gradients. A detachment algorithm based on structure similarity constraint was proposed to separate the two echo signals. ${\rm{T}}_{{2}}$ mapping was obtained from the two separated echo signals. Results: The robustness and efficiency of the method were demonstrated through simulation, phantom experiments, and human brain measurements. Conclusion: Reliable ${\rm{T}}_{{2}}$ mapping can be obtained within milliseconds even under continuous head motion. Significance: Reliable ${\rm{T}}_{{2}}$ mapping was achieved with a single shot for the first time. The proposed method will facilitate real-time dynamic and quantitative MR imaging.