The avalanche transistor-based Marx circuit is often used to generate high-voltage nanosecond pulses, its output waveform usually has a rising time about hundreds of picoseconds, an exponential discharging falling edge, and an output voltage in kV-level. However, the typical output waveform falling edge of this circuit structure usually has oscillation or distortion. Meanwhile, as long as the main capacitance of Marx circuit is large enough, the spike oscillation would emerge in the rising edge of the output waveform. Previous studies have paid less attention to this or attributed it to the influence of circuit stray parameters and impedance matching. In this paper, the simulation analysis was carried out from the perspective of the dynamic switching process of the avalanche transistor, and the influences of the main capacitor, the number of Marx circuit stages and the charging voltage were studied experimentally. The results show that the operating state of the avalanche transistor in voltage ramp mode caused the waveform oscillation. The oscillation would be more obvious when the circuit has larger main capacitance, fewer Marx stages and lower charging voltage. Moreover, the amplitude of the pulse oscillation could even be higher than the fast front edge, at this time, the fast front edge could be regarded as the spike oscillation in the output waveform rising edge. The output waveform oscillation can be reduced by adjusting the main capacitance of Marx circuit and optimizing the structure of microstrip line.