By the extensive tensor network algorithms, we provide the phase diagram of the frustrated J1-J2 Ising model on the square lattice. Based on the fine numerical simulations, we discuss the cases with controversy in the phase diagram, especially for the stripe transition in the regime g = |J2/J1| > 1/2, (J2 > 0, J1 < 0). Our simulations suggest that the first-order phase transition may reside in a very narrow parameter region 1/2 < g < g * (g * is much smaller than previous research). Combining with the analysis of critical properties, we find the evidence that the classical J1-J2 model evolves continously from two decoupled Ising models (g → ∞ with central charge c = 1) to a point of tricritical Ising model (with c = 0.7) as g decreases to g * .