A large number of observation data about BL Lac objects are collected in the compilation. On the basis of this, I analyse and generalize all of the characteristics of BL Lac objects, introduce two kinds of models, i.e. accretion disk and jet. in particular, discuss the coherent collective emission of electromagnetic waves from plasmajet. Using the S-shaped limit cycle around supermassive blackhole, we explain the quasi-periodic short timescale outburst of BL Lac object On + 231. Conclusion is drawn that the accretion of blackhole is the possible energe source of ON + 231. The correlations of radio, optical and X-ray luminosity are examined for the X-ray selected (XS) and radio selected (RS) BL Lac objects. It is found that strong correlations appear between any two monochromatic luminosities for RS objects, however, only the radio and optical are correlated for XS objects. Considering the relativistic effect of jet, we correct the observated flux with Doppler factor δ~(3+α) for RS objects, more strong correlation present. The results are consistent with two components origin for X-ray emission, one is isotropic and one relativistically beamed, the former is dominant intrinsically but the latter dominates observationally. In addition, two open quetions about gravitation lense and relation of BL Lacs and QSOs are discussed.
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