A connecting interface unit and a memory storage device without a crystal oscillator are provided and include following circuits. A phase detector detects a phase difference between a first reference signal and an input signal from a host system to generate a phase signal. A signal detecting circuit detects a signal character difference between the input signal and the first reference signal for a signal generating circuit to generate a second reference signal. A phase interpolator generates a clock signal according to the phase signal and the second reference signal. A sampling circuit generates an input data signal according to the clock signal. A transmitter circuit modulates an output data signal according to the clock signal or the second reference signal to generate an output signal, and transmits it to the host system. Accordingly, the connecting interface unit conforms to the specification of a transmission standard.