In this paper, we classify the hypersurfaces of S 2 × S 2 with constant sectional curvature. By applying the so-called Tsinghua principle, which was first discovered by the first three authors in 2013 at Tsinghua University, we prove that the constant sectional curvature can only be 1 2 and the product angle function C defined by Urbano is identically zero. We show that any such hypersurface is a parallel hypersurface of a minimal hypersurface in S 2 × S 2 with C = 0, and we establish a one-to-one correspondence between the involving minimal hypersurface and the famous "sinh-Gordon equation"As a byproduct, we give a complete classification of the hypersurfaces of S 2 × S 2 with constant mean curvature and constant product angle function C.