Recently Vadim Lyubashevsky etc. built LWE problem on ring and proposed a public cryptosystem based on R-LWE, which, to a certain extent, solved the defect of large public key of this kind, but it didn't offer parameter selections and performance analysis in detail. In this paper an improved scheme is proposed by sharing a ring polynomial vector that makes public key as small as 1/m of the original scheme in multi-user environments. In additions, we introduce a parameter r to control both the private key space size and decryption errors probability, which greatly enhances the flexibly and practicality. The correctness, security and efficiency are analyzed in detail and choice of parameters is studied, at last concrete parameters are recommended for the new scheme.