The investigations of interaction processes between ion beams and gas and between ion beams and plasma play important roles in atomic physics, astrophysics, high energy density physics, and inertial confinement fusion.The atomic density of target is one of the key experimental parameters which may determine the interaction mechanism and experimental results.How to precisely diagnose the atomic density of target in different matter states, like gas phase and plasma phase, is challenging work on the experiments in laboratory.Conventionally the vacuum gauges are used to measure the pressure inside the gas target, but the accuracy is limited for a complex target system and they can hardly work in a strong radiation surrounding, especially in plasma where the high temperature can physically damage the gauges.Therefore we propose a new method to measure the atomic densities for both gas target and plasma target based on the heavy ion beam accelerator facility at the Institute of Modern Physics, Chinese Academy of Sciences.In our experiment the protons are extracted from an electron cyclotron resonance ion source (ECRIS) and accelerated to 100 keV then transmitted to the target.A two-stage differential pumping system is constructed to keep 10-7 mbar order of magnitude in beam line when the gas is filled into the target area where the pressure could increase to higher than 1 mbar.A 45 dipole magnet is used to bend the protons which have passed through the gas.The energy is determined by the different positions of protons at the position-sensitive detector which is placed at the end of magnet.Consequently the energy losses of protons at different pressures are obtained.There have been proposed many theories for calculating the energy loss of protons in gas, and we chose the very popular code named SRIM to simulate the experimental case. Finally the effective linear atomic density of target along the ion beam trajectory in the target area is obtained.For comparison, the conventional vacuum gauges (one is the hot cathode gauge-IonIVac ITR 90 and the other is capacitance diaphragm gauge-Varian CDG-500) are simultaneously used in the experiment.The results show that the recalibrated effective pressure obtained by the energy loss is close to the pressure measured by Varian CDG-500 but much lower than the pressure from IonIVac ITR 90.Only after the detection efficiency correction, could the corrected results of IonIVac ITR 90 be coincident with the effective pressure obtained according to energy loss.Moreover we find that the effective atomic density determined by the protons energy loss shows that these advantages over the conventional gauges are not only the high accuracy and reliability but also the in-situ measurement, high temporal resolution and the ability to work in the complex radiation and hot plasma environment.These properties may play a great role in the experimental researches and relevant topics.