An eye opening monitor device and an operation method thereof are provided. The eye opening monitor device includes a phase interpolator, a first sampling circuit, a second sampling circuit, and a clock centering circuit. The first sampling circuit samples a data signal according to a data clock to generate first sampled data. The second sampling circuit samples the data signal according to a phase interpolation clock to generate second sampled data. The phase interpolator changes a phase of the phase interpolation clock according to a phase interpolation code. The clock centering circuit counts multiple comparison results of the first sampled data and the second sampled data in multiple clock cycles to obtain an error count value for any one of different phase interpolation codes. The clock centering circuit determines the phase interpolation code provided to the phase interpolator based on the error count values corresponding to different phase interpolation codes.