Observational equivalence is a powerful means for formulating the security properties of cryptographic protocols. However suffering from the infinite quantifications over contexts, its proof becomes notoriously troublesome. This paper addresses the problem with a symbolic technique. We propose a symbolic bisimulation for spi calculus based on an environmental sensitive label transition system semantics, which restrict the infinite inputs of a process to only finite transitions. We also prove that the symbolic bisimulation is sound to the traditional concrete bisimulation, and furthermore is a promising means to automatically verifying the security protocols.