The invention relates to an improved microporous polymer film and a preparation method thereof, and belongs to the field of chemistry. The improved microporous polymer film comprises the following compositions by weight percentage: 55% to 99.79% of polypropylene homopolymer, 0.01% to 5% of beta crystal nucleating agent, 0.1% to 20% of polymeric additive and 0.1% to 20% of inorganic modifier. The improved microporous polymer film is prepared by stretching a melt about 2 to 60 times at a speed of 1 m to 100 m / min in casting, calendering, extruding or film blowing process. Compared with the prior art, more beta crystals can be obtained within shorter time, the formed beta crystals have higher thermal stability, and have better film-forming stability and pore-forming properties, and the prepared microporous film has the characteristics of easiness in processing, higher permeability, excellent productivity, and strong mechanical properties and dimensional stability, and the porosity is further improved, and thereby the permeability of various media can be obviously improved.