In this paper, we consider joint delay-Doppler estimation in a joint radar and communication (JRC) system based on Orthogonal time frequency space (OTFS) modulation. The proposed scheme use the atomic norm to combat the off-grid problem in discrete domain. Unlike previous works that assume the target locates right on the grid point, this work estimates the delay and Doppler frequency in continuous domain. By introducing the atomic norm minimization theory, the problem is converted into a semidefinite program (SDP). To solve this SDP in an iterative pattern, we also develop the alternating direction method of multipliers (ADMM). Numerous simulation results are presented to demonstrate the superior performance of the proposed method in comparison with the state-of-the-arts.