The potential in synaptic simulation for neuromorphic computation has revived the research interest of resistive random access memory (RRAM). However, novel applications require reliable multi-level resistive switching (RS), which still represents a challenge. We demonstrate in this work the achievement of reliable HfO2-based RRAM devices for synaptic simulation by performing the Al doping and the post-deposition annealing (PDA). Transmission electron microscopy and operando hard X-ray photoelectron spectroscopy results reveal the positive impact of Al doping on the formation of oxygen vacancies. Detailed I-V characterizations demonstrate that the 16.5% Al doping concentration leads to better RS properties of the device. In comparison with the other reported results based on HfO2 RRAM, our devices with 16.5% Al-doping and PDA at 450°C show better reliable multi-level RS (~20 levels) performance and an increased On/Off ratio. The 16.5%Al:HfO2 sample with PDA at 450°C show good potentiation/ depression characteristics with low pulse width (10 s) along with good On/Off ratio (>1000), good data retention at room temperature and high temperature and good program/erase endurance characteristics with a pulse width of 50 ns. The synapse features including potentiation, depression and spike time dependent plasticity (STDP) were successfully achieved using optimized Al-HfO2 RRAM devices. Our results demonstrate the beneficial effects of Al doping and PDA on the enhancement of the performances of RRAM devices for the synaptic simulation in neuromorphic computing applications.