The key for using a hyperbolic model to describe stress-strain relationship of liquefiable soil is to determine its cyclic maximum shear modulus and its cyclic ultimate shear stress under cyclic loading. Aiming at several kinds of sand soil with different relative densities, liquefaction tests under the action of cyclic stresses of different equal-amplitudes were uniformly consolidated using a new high-precision dynamic tri-axial apparatus to study effects’ mode and law of pore-water pressure on maximum shear modulus and ultimate shear stress of saturated sand soil. Formulas to calculate cyclic maximum shear modulus and ultimate shear stress of sand soil with different precisions were proposed considering increase in pore-water pressure. The results showed that effects of increase in pore-water pressure on cyclic maximum shear modulus and ultimate shear stress of sand soil are obvious, sand soil’s cyclic maximum shear modulus and ultimate shear stress decrease with increase in pore-water pressure; the relation between saturated sans soil’s maximum shear modulus and pore water pressure ratio can be expressed as a unified linear relation expression to be independent upon sand soil types and their relative densities, pore water pressure ratio is equal to the relative reduction of cyclic maximum shear modulus; the relation between cyclic ultimate shear stress of saturated sand soil and pore water pressure ratio can accurately be expressed as a quadratic curve relation to be dependent on sand soil types and their relative densities, it also can be expressed as a unified linear relation expression to be independent upon sand soil types and their relative densities considering simplification, pore water pressure ratio is equal to the relative reduction of cyclic ultimate shear stress; Hardin initial maximum shear modulus calculation formula is not suitable for calculation of cyclic maximum shear modulus considering increase in pore water pressure, Hardin formula generally overestimates sand soil’s cyclic maximum shear modulus in liquefaction process; especially, the cyclic maximum shear modulus can be overestimated almost 80%-140% within the sensitive interval of pore water pressure ratio of 0.6 ~0.8.