The invention discloses a porous polyvinylidene fluoride diaphragm used for a lithium ion battery and a preparation method thereof. The porous polyvinylidene fluoride lithium ion battery diaphragm comprises the following components in parts by weight: 20-70 parts of one type of polyvinylidene fluoride resin or at least more than two of a polyvinylidene fluoride resin mixture, and 30-80 parts of adiluent. The preparation method comprises the following steps: 1) raw material pretreatment; 2) mixing; 3) granulation; 4) drying; 5) dried particles casting through a casting line for film forming; 6) transverse stretching or vertical stretching or transverse and vertical stretching on a film; and 7) extraction on the stretched film through an extractant to obtain the porous polyvinylidene fluoride lithium ion battery diaphragm. The porous polyvinylidene fluoride lithium ion battery diaphragm can resist acid-base solutions with pH values from 1 to 14, and cannot be dissolved in an organic solvent such as alcohol, aldehyde and benzene; wettability of an electrolyte is good, and thermal contraction resistance is good. The preparation method has the advantages of simple process, environmental protection, and high production efficiency.