To explore the influence of pore diameter and constraint degree on cracking time and crushing effect in static crushing process, static crushing tests of 13 plain concrete specimens were carried out. The ratio of concrete area in inscribed circle of test specimen to crushing agent area was defined as constraint ratio, which was used to express the constraint degree of concrete to crushing agent. Volume expansion rate of crushing agent was utilized to indicate the crushing effect of specimens. Results show that: 3 or 4 cracks appeared after the broken of the concrete blocks under a single hole, and 3 cracks were the commonest, when the cracks became stable, the distribution forms were in the shapes of “Y”, “T”, and “cross”; The time-history curve of the volume expansion rate of crushing agent after cracking was in the form of quadratic parabola, which developed rapidly in early stage, slowly in later stage, and gradually became stable; Both pore diameter and constraint degree had obvious effects on volume expansion rate of crushing agent, the larger the pore diameter was, the larger the volume expansion rate of crushing agent became, and the better the crushing effect was, as constraint ratio increased, volume expansion rate of crushing agent decreased, and crushing effect was weakened; When constraint ratio was small, pore diameter had major effects on cracking time, where the larger the pore diameter was, the shorter the cracking time became, when constraint ratio was large, constraint ratio had major effects on cracking time, where the larger the constraint ratio was, the longer the cracking time became.