The purpose of the ALFA detector is to measure protons scattered at very small angles. ALFA aims to study elastic and diffractive events, exclusive production and photon induced interactions. In LHC Run 2, this detector participated in the ATLAS high-luminosity data taking. In addition, several special runs with reduced luminosity were taken. Any cross section measurement requires good understanding of the particle reconstruction efficiency. This task is particularly complicated in the case of forward protons where the actual beam condition is an important part of the working environment. The modelling of the very forward region in terms of primary particle flux and inactive material producing secondary particle is generally not precise. An overlay technique of Monte Carlo signal events with zero-bias collider data has overcome these difficulties and may provide better understanding of the proton reconstruction efficiency. This report presents results of the proton reconstruction efficiency with the ALFA detector during LHC Run 2.