The acceleration of electrons in 3C 279 is investigated through analysing the injected electron energy distribution (EED) in a time-dependent synchrotron self-Compton+external Compton emission model. In this model, it is assumed that relativistic electrons are continuously injected into the emission region, and the injected EED [Q(e)'(gamma')] follows a single power-law form with low- and high-energy cut-offs gamma(min)' and gamma(max)', respectively, and the spectral index n, i.e. Q(e)'(gamma') proportional to gamma'(-n). This model is applied to 14 quasi-simultaneous spectral energy distributions of 3C 279. The Markov Chain Monte Carlo (MCMC)-fitting technique is performed to obtain the best-fitting parameters and the uncertainties on the parameters. The results show that the injected EED is well constrained in each state. The value of n is in the range of 2.5-3.8, which is larger than that expected by the classic non-relativistic shock acceleration. However, the large value of n can be explained by the relativistic oblique shock acceleration. The flaring activity seems to be related to an increased acceleration efficiency, reflected in an increased gamma(min)' and electron injection power.