The aim of our observations is to investigate the intra-night variability properties and the spectral variability of BL Lacertae. 799 optical multiband observations were intensively made with the Beijing-Arizona-Taiwan-Connecticut (BATC) 60/90cm Schmidt telescope during the outburst composed of two subsequent flares in 2005. The second flare, whose rising phase lasted at least 44d, was observed with amplitudes of more than 1.1mag in three BATC optical bands. In general, on intra-night time-scale the amplitude of variability and the variation rate are larger at the shorter wavelength, and the variation rate is comparable in the rising and decaying phases within each band. A possible time-lag between the light curves in bands e and m, around 11.6min, was obtained. Based on the analysis of the colour index variation with the source brightness, the variability of BL Lacertae can be interpreted as having two components: a 'strongly chromatic' intra-night component and a 'mildly chromatic' inter-night component, which may be the results of both intrinsic physical mechanism and geometric effects.