Phase-sensitive optical time domain reflectometry (Φ-OTDR) realizes quantitative measurement of the dynamic strain employing phase demodulation. Unfortunately, it is difficult to measure the large dynamic strain with the conventional Φ-OTDR due to the restriction of the unwrapping algorithm. In this work, an approach based on two-wavelength probe is proposed and demonstrated to improve the measurable range of the dynamic strain in Φ-OTDR. By utilizing the difference between the two phases acquiring with two different lasers, the large dynamic strain can be recovered. In experiments, dynamic strains with peak values from 10.32 uɛ to 24.08 uɛ are retrieved accurately, which cannot be recovered with the conventional Φ-OTDR. Moreover, the tunable sensitivity is also demonstrated through adjusting the wavelengths of the probe. With the increment of the wavelength interval from 9.06 nm to 23.06 nm, the normalized sensitivity increases from 0.4 to 1 accordingly. That agrees well with the theoretical prediction. Foreseeably, the proposed method will extend the scope of application fields for Φ-OTDR, which requires large dynamic strain recognition.