The invention provides an organic / inorganic composite porous isolating membrane, a preparation method thereof and an electrochemical device. According to the first aspect of the invention, the organic / inorganic composite porous isolating membrane comprises a porous base material and an organic / inorganic composite porous coating coated on at least one surface of the porous base material, wherein the organic / inorganic composite porous coating comprises inorganic particles, a binding agent and organic particles with at least two swelling degrees, and the organic particles are swelled by a plasticizer. The preparation method of the organic / inorganic composite porous isolating membrane is used for preparing the organic / inorganic composite porous isolating membrane described in the first aspect of the invention. According to the third aspect of the invention, the electrochemical device comprises the organic / inorganic composite porous isolating membrane described in the first aspect of the invention. Therefore, the organic / inorganic composite porous isolating membrane can form an excellent interface, so that a risk that the risk of blocking holes by the organic particles is reduced, breathability of the organic / inorganic composite porous isolating membrane is improved, and electrical conductivity of the organic / inorganic composite porous isolating membrane is improved.