We present extensive optical photometric and spectroscopic observations for the nearby Type Ia supernova (SN Ia) 2019ein, spanning the phases from similar to 3 to similar to 330 d after the explosion. This SN Ia is characterized by extremely fast expansion at early times, with initial velocities of Si ii and Ca ii being above similar to 25 000-30 000 km s(-1). After experiencing an unusually rapid velocity decay, the ejecta velocity dropped to similar to 13 000 km s(-1) around maximum light. Photometrically, SN 2019ein has a moderate post-peak decline rate (Delta m(15)(B) = 1.35 +/- 0.01 mag), while being fainter than normal SNe Ia by about 40 per cent (with M-B(max) approximate to -18.71 +/- 0.15 mag). The nickel mass synthesized in the explosion is estimated to be 0.27-0.31 M-circle dot from the bolometric light curve. Given such a low nickel mass and a relatively high photospheric velocity, we propose that SN 2019ein likely had a sub-Chandrasekhar-mass white dwarf (WD) progenitor, M-WD less than or similar to 1.22 M-circle dot. In this case, the explosion could have been triggered by a double-detonation mechanism, for which 1D and 2D models with WD mass M-WD approximate to 1 M-circle dot and a helium shell of 0.01 M-circle dot can reasonably produce the observed bolometric light curve and spectra. The predicted asymmetry as a result of double detonation is also favoured by the redshifted Fe II and Ni II lines observed in the nebular-phase spectrum. Possible diversity in origin of high velocity SNe Ia is also discussed.