The distribution of a backfill grouting layer behind concrete lining of a shield tunnel is important for evaluation of its stability. In this paper, we proposed to use Common Mid-Point (CMP) method by Ground Penetrating Radar (GPR) to estimate the dielectric permittivity and thickness of the backfill grouting layer. We developed a CMP measurement system with two designed bowtie antennas. Field experiments were carried out in two shield tunnels in Shanghai, China in January 2012. One is the Yangzi River tunnel under operation and the other is the Changjiang Road tunnel under construction. Two datasets were acquired in each tunnel. Frequency wavenumber (FK) filter was applied to suppress the coherent clutter and enhance the hyperbolic reflection signal. A new algorithm of envelope velocity spectrum analysis was proposed for accurate estimation of dielectric permittivity and thickness of the grouting layer from processed CMP profiles. Both the grouting layers behind the tunnel under operation and that under construction can be clearly estimated. The thickness of grouting layer behind the investigated two segments in Yangzi River tunnel were respectively estimated to be 16 cm and 17 cm, a little less than the designed thickness of 20cm. In addition, air void was discovered inside the grouting layer behind one of the lining in Changjiang Road tunnel. This technique provides important information for evaluation of the safety of a tunnel, mitigation of the risk of ground settlement.