A power-tunable local oscillator (LO) is utilized for long-distance phase-sensitive optical time-domain reflectometry (ϕ-OTDR) systems. The scheme uses a LO whose power is intentionally varied inversely with the backscattered signal power to suppress the dynamic signal range before receiving, which realizes a higher far-end signal-to-noise ratio (SNR) while avoiding receiver power saturation. Meanwhile, the suppressed signal range allows for the use of low-resolution analog-to-digital converters (ADC). We implemented the scheme over a 98-km standard single-mode fiber, and the results show that the scheme achieves a 3.4 dB far-end SNR improvement. When the ADC resolution decreases from 13 to 8 bits, the proposed scheme shows a negligible SNR penalty. Compared to systems using power-fixed LO and 8-bit ADCs, our proposed scheme achieves a 4.8-dB reduction in self-phase noise power under the same conditions.