Environmental risk is ascribed to contamination level as well as the potential decontamination capability of the contaminated sites. In some cases, natural attenuation by biodegradation may dominate self cleanup as microorganisms always try to survive, which reduces risk to human health. Conventional assessment is based on sequential monitoring of concentrations, which requires a large number of samples and long time. In addition, some organic compounds like polycyclic aromatic hydrocarbons(PAHs) are hard to be accurately measured due to their hydrophobic properties. The paper presented a new method to evaluate risk alleviation of PAHs contaminated sites. Instead of conventional method, the new method proposed DNA copies/cell quantity ratio as an indicator to evaluate microbial activity which indirectly reflects the trend of risk alleviation. The concept was theoretically elaborated and verified by lab experiments. In this study, Mycobacterium vanbaalenii PYR-1, a typical PAHs degrading bacterium, was introduced and phenanthrene was used as the sole PAHs substrate. The DNA copies and cell quantity were determined by real time polymerase chain reaction (PCR) and colony forming units (CFU) method respectively. Conventional method was also carried out as controls. The results implied the new method may be a fast evaluation of risk alleviation and potentially supportive for engineering applications.