In recent years, secure multiparty computation is one of research focuses in the field of cryptography, and secret geometry calculation is an important branch of it. The problem of safely calculating a straight line by two private coordinate point has important application prospect in space information security. First, a variant of Paillier's homomorphic encryption scheme is put forward in that the base is calculated by sender during encryption, and its indistinguishability under adaptive chosen-plaintext attack is proved. Then, based on this homomorphic encryption scheme, a protocol that can safely calculate a straight line by two private coordinate point in semi-honesty model is designed. Moreover, this protocol can be applied to solve a type of secure multiparty computational geometry problem that can be reduced to compute coordinate difference quotient. Thus, the problem that there is a non-negligible probability of private information leakage in the current coordinate difference quotient calculation protocols based on homomorphic encryption is solved.