In this work, a series of polyethylene/poly(ethylene-co-propylene) in-reactor alloys (PE/EPR) were synthesized with MgCl2-supported Ziegler-Natta catalyst by a three-stage reaction process including slurry ethylene homopolymerization, gasphase ethylene/propylene copolymerization and alternating homo- and copolymerization (PSPP stage), and the effects of switching frequency (SF) on their structure and properties were studied. The alloy was composed of random ethylene/propylene copolymer (EPR), segmented ethylene/propylene copolymer (EPS) and polyethylene (PE). Increasing SF in the PSPP stage led to significant changes in the composition, chain structure, mechanical properties and phase morphology of the alloys. By increasing SF from 1 to 30, the amount of EPR fraction slightly decreased then increased, but the EPS fraction significantly increased in a monotonous way. Increasing SF led to formation of EPS with longer PE sequence and higher crystallinity. The alloys showed much higher toughness at low temperature (-50 °C) than polyethylene. The alloy prepared at SF=6 showed better toughness-stiffness balance than those prepared under other conditions. The PE/EPR alloys showed typical sea-islands phase structure, with the EPR domains being the dispersion phase. The alloy prepared at SF = 6 showed EPR phases with the smallest size and the most uniform spacial distribution in the PE matrix. The observed improvements in alloys mechanical properties are explained by the enhancement of phase compatibility with increase in SF.