On the basis of the n-acenes (n = 1, 2, 3 and 4), the alpha-Li@n-acenes and beta-Li@n-acenes salts were selected to investigate how increasing the number n of conjugated benzenoid rings affects the linear and nonlinear optical responses. The alpha-Li@n-acenes and beta-Li@n-acenes salts are obtained by a lithium atom substituting the alpha-H and beta-H, respectively. In the present work, both ab initio (HF and MP2) and DFT (B3LYP, BhandHLYP, M05-2X, and CAM-B3LYP) methods are adopted to calculate the polarizability (alpha(0)) and first hyperpolarizability (beta(tot)) of the alpha-Li@n-acenes and beta-Li@n-acenes salts. MP2 results show that the alpha(0). values of both classes of lithium salts increase with increasing number n of conjugated benzenoid rings. Interestingly, we found that the beta(tot) values of alpha-Li@n-acenes and beta-Li@n-acenes salts take on opposite trends: the beta(tot) values of alpha-Li@n-acenes are decreasing slowly (2187 for alpha-Li@benzene > 1978 for alpha-Li@naphthalene > 1898 for alpha-Li@anthrecene > 1830 au for alpha-Li@tetracene) and inceasing remarkably (2738 for beta-Li@naphthalene < 3186 for beta-Li@anthrecene < 3314 au for beta-Li@tetracene) for beta-Li@n-acenes. Furthermore, we found that the beta(tot) values (2738-3314 au) of the beta-Li@n-acenes are larger than those of the alpha-Li@n-acenes (1830-2187 au). On the other hand, comparing the results of different methods, the beta(tot) values obtained by the M05-2X and CAM-B3LYP methods reproduce the polarizability and first hyperpolarizability of the alpha-Li@n-acenes and beta-Li@n-acenes salts well, which test and verify the results of the MP2 method. Our present work may be beneficial to development of high-performance organic NLO optical materials.