Incorporating waste glass powder (WGP) into concrete as supplementary cementitious materials (SCMs) offers a potential method for recycling glass. To facilitate the effective application, it is necessary to research the effect and mechanism of waste glass on mortar properties. In this paper, the effect of WGP with different particle sizes of 0~0075 mm, 0.075~0.15 mm and 0.15~0.3 mm as SCMs on the mechanical properties of mortar and expansion of alkali-silica reaction (ASR) was studied. The results show that the 28 d compressive strength of mortar increases by 5%~15% and ASR expansion rate decreases by 20.2% when the size of WGP is 0~0.075 mm. The 28 d compressive strength of mortar decreases by 5%~8% and the ASR expansion rate increases by 39.7% when the size of WGP is 0.15~0.3 mm. To further explore the mechanism of performance change, thermogravimetric analysis, inductively coupled plasma and electron microscope and energy dispersive spectrometer were used to examine the resulting products, pore solution, microstructure and composition. The alkali aggregate activity of coarse WGP is strong, easy to ASR, resulting in ASR expansion rate increased. The pozzolanic reactivity of fine WGP is strong, the pozzolanic reaction produces calcium silicate hydrate gel with a low Ca/Si ratio and a low expansion rate. The product absorbs Na+ and K+, reducing the number of reactants needed for ASR, and the resulting mortar is denser than the plain mortar, which is beneficial for reducing porosity, reducing water infiltration, and resisting expansion pressure.