Pressure variations inside a fuel injection system can lead to fluctuations in injected fuel quantity during injections and therefore can directly impact the stability and performance coherence of the fuel injection system and the engine it matched. Concepts of hydraulic capacitance and hydraulic inductance are introduced into fuel injection system by referencing LC oscillator electric circuit. A LC undamped hydraulic mathematical model is developed based on the structural parameters of the electronic unit pump (EUP). Pressure fluctuation frequency during injection process at different operating conditions of the EUP is computed by the LC undamped model. It is observed that the frequencies of pressure fluctuations tend to increase with the fuel injection process. Pressure curves and pressure fluctuation frequencies obtained by experiments at different operating conditions validated the results predicted by the LC undamped mathematical model.