In the past we developed an injection-rate detector, however, it becomes no more applicable to modern high-pressure piezoelectric injection systems having functions of multi-stage injection due to the following problems: High-pressure injection generates shockwaves that induce pressure fluctuation, whose amplitudes of high-frequency components could be not effectively attenuated with a low-pass filter, in the detector. High-pressure injection also causes heterogeneous distribution of temperature in the detector, because the pattern of fuel flow from the injection nozzle to the discharge valve at fuel-discharging process is inappropriate. Accordingly, fuel temperature, which is necessary for identifying bulk modulus of fuel, in the detector could not be precisely obtained, thereby causing an unacceptable level of scattering for determining injection quantity. Hence, we developed a new detector in modifying its constructive design to solve the problems. As a result, multi-stage injection-rate and injection quantity can be more precisely measured.