Squeezing of quantum fluctuation plays an important role in fundamental quantum physics and has marked influence on ultrasensitive detection. We propose a scheme to generate and enhance the squeezing of mechanical mode by exposing the optomechanical system to a non-Markovian environment. It is shown that the effective parametric resonance term of mechanical mode can be induced due to interaction with the cavity and non-Markovian reservoir, thus resulting in quadrature squeezing of the mechanical resonator; jointing the two kinds of interactions can enhance the squeezing effect. Compared with the usual Markovian regime, we can obtain stronger squeezing, and, significantly, the squeezing can approach a low asymptotic stable value.