To precisely reveal the relation between fuel injection quantity and characteristic parameters defined from the fluctuation law of instantaneous rail pressure of high pressure common rail fuel injection system of diesel engines， a fuel injection quantity observation （IQO） method was proposed based on the characteristic parameters of instantaneous rail pressure waveforms. With this method， the instantaneous rail pressure waveforms were divided into a series of typical segments. Four characteristic points and four characteristic segments were defined to describe the instantaneous rail pressure fluctuation characteristics of each cylinder. And ten characteristic parameters were defined to evaluate the instantaneous rail pressure fluctuation characteristics in the fuel injection process of each cylinder. A database of the characteristics of rail pressure waveform groups was set up and the fluctuation law of the instantaneous rail pressure under different operating conditions was revealed by mean processing of the characteristic parameters of the instantaneous rail pressure fluctuation at every steady experimental operating point. An IQO model was established by the fuel continuity equations， and calibrated by the parameter identification and offline validation with the characteristics database of rail pressure waveform groups. The results show that the IQO model has simple structure and high observation accuracy and it lays a good foundation for further engineering application in electronic control unit （ECU）.