We investigate the optical spectral properties of the blazar PKS 1222+216 during a period of approximately three years. While the continuum is highly variable [i.e. from λLλ(5100 Å) ∼ 3.5 × 1045 erg s−1 up to ∼15.0 × 1045 erg s−1] the broad line emission is practically constant. This supports a scenario in which the broad line region is not affected by jet continuum variations. We thus infer the thermal component of the continuum from the line luminosity and show that it is comparable with the continuum level observed during the phases of minimum optical activity. The mass of the black hole is estimated through the virial method from the full width at half-maximum of Mg ii, Hβ and Hα broad lines and from the thermal continuum luminosity. This yields a consistent black hole mass value of ∼6 × 108 M⊙.