The invention provides a method and device for acquiring the porosity of sandstone. The method includes the following steps that firstly, the elasticity bulk modulus, elasticity shear modulus, viscoelasticity shear modulus and viscoelasticity shear coefficient of the sandstone are acquired; secondly, the vertical stress, maximum main horizontal stress and minimum main horizontal main stress borne by the sandstone are acquired; thirdly, the bulk strain of the sandstone is acquired according to the elasticity bulk modulus, elasticity shear modulus, viscoelasticity shear modulus, the viscoelasticity shear coefficient, vertical stress, maximum main horizontal stress and minimum main horizontal main stress of the sandstone; finally, the porosity of the sandstone is acquired according to initial porosity of the sandstone and the bulk strain of the sandstone. According to the method and device for acquiring the porosity of the sandstone, no drilling is needed for acquiring a sandstone sample when the porosity of the sandstone is acquired, and the cost for the porosity of the sandstone is reduced.