This brief reports a universal stability criterion for the type-I sampling phase-locked loop (S-PLL). The linear-time- variant model predicts with precision a maximum phase margin that decreases linearly with the increasing loop bandwidth (BW), leading to a maximum achievable loop BW that is half of the reference frequency. We verified the derived theory through extensive simulations and will present a guide that is useful in the design and optimization of different S-PLL schemes.