We investigate the transition from the many-body localized phase to the ergodic thermalized phase at an infinite temperature in an XY spin chain with L spins, which experiences power-law decaying interactions in the form of Vij ∝ 1/ |i − j| α (i, j = 1, · · · , L) and a random transverse field. By performing large-scale exact diagonalization for the chain size up to L = 18, we systematically analyze the energy gap statistics, half-chain entanglement entropy, and uncertainty of the entanglement entropy of the system at different interaction exponents α. The finite-size critical scaling allows us to determine the critical disorder strength Wc and critical exponent ν at the many-body localization phase transition, as a function of the interaction exponent α in the limit L → ∞. We find that both Wc and ν diverge when α decreases to a critical power αc 1.16 ± 0.17, indicating the absence of many-body localization for α < αc. Our result is useful to resolve the contradiction on the critical power found in two previous studies, αc = 3/2 from scaling argument in Phys. Rev. B 92, 104428 (2015) and αc ≈ 1 from quantum dynamics simulation in Phys. Rev. A 99, 033610 (2019). arXiv:1908.04031v2 [cond-mat.quant-gas]