An eye diagram measuring method includes: sampling a compensated input signal according to a reference voltage and a reference clock to obtain a first sampling result; and sampling a to-be-compensated input signal according to a scan voltage and a scan clock to obtain a second sampling result, including: (b1) storing a minimum phase and a voltage level which render the first sampling result identical to the second sampling result; (b2) increasing the voltage level and repeating operation (b1); (b3) decreasing the voltage level and repeating operation (b1); (b4) storing a maximum phase and the voltage level which render the first sampling result identical to the second sampling result; (b5) increasing the voltage level and repeating operation (b4); and (b6) decreasing the voltage level and repeating operation (b4). Voltage levels, maximum phases and minimum phases that are stored are for adjusting the reference voltage and the reference clock.