Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) towards hydrogen evolution reaction. First, ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mM) under helium atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under He atmosphere shows that each discrete current profile of collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen-containing atmospheres. The particle size distribution estimated from the diffusion-controlled current in He atmosphere agrees well with both the electron microscopy and DLS observations. The work presented herein sheds lights on the interfacial dynamics of the nanoparticle collision electrochemistry.