We present optical observations of the Type IIb supernova (SN) 2017ckj, covering approximately 180 days after the explosion. Its early-time multi-band light curves display no clear evidence of a shock-cooling tail, resembling the behavior of SN2008ax. The V-band light curve exhibits a short rise time of about 5 days and reaches an absolute peak magnitude of M_V_=-18.58+/-0.17mag. The late-time multi-band light curves reveal a linear decline. We modelled the bolometric light curve of SN2017ckj to constrain the progenitor and the explosion parameters. We estimated a total mass of ^56^Ni synthesized by SN2017ckj of M_Ni_=0.21^+0.05^-0.03_M{sun}, with a massive H-rich envelope of M_env=0.4^+0.1^-0.1_M{sun}. Both the ^56^Ni mass and the envelope mass of SN2017ckj are higher than those of typical SNe IIb, in agreement with its peculiar light curve evolution. The early-time spectra of SN2017ckj are dominated by a blue continuum, accompanied by narrow H_alpha and HeII emission lines. The earliest spectrum exhibits flash ionization features, from which we estimated a progenitor mass-loss rate of ~3.4x10^-4^M_{sun}/yr. At later epochs, the spectra develop broad P-Cygni profiles and become increasingly similar to those of SNe IIb, especially SN2018gk. The late-time spectrum at around 139 days does not show a distinct decline in the strength of H_alpha emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the H_alpha_ feature, the nebular spectrum exhibits prominent emission lines of OI, CaII, [CaII], and MgI], which are consistent with the prototypical SN1993J.
