An eye-width detector, a memory storage device and an eye-width detection method of data signal are provided. The eye-width detector includes a phase interpolator, a calibration circuit and an eye-width detection circuit. The phase interpolator receives a first clock signal and a phase control signal and output a second clock signal. The calibration circuit receives the first clock signal and the second clock signal and output a first control signal. The eye-width detection circuit receive the data signal, the first clock signal and the second clock signal and generate a first sampling value and a second sampling value. If the first sampling value and the second sampling value do not match a first condition, the eye-width detection circuit outputs a second control signal; otherwise, outputs eye-width information of the data signal. Accordingly, the efficiency of the eye-width detection may be improved.