Guided by the solidification of loess contaminated with heavy metal ions (HMs), a natural inorganic diatomite (NID) was developed as curing agent under an alkaline activator (AA). The curing time, NID content and AA type on the mechanical properties of contaminated soil and solidification effect of HMs were investigated. The solidification source was analysed by microstructure measurement. As curing time increased, the solidification effect increased, with an optimum curing time of 28 days. The higher the content of NID, the stronger the solidification ability. Nevertheless, the strength showed a tendency of initial increase and subsequent decrease. The strength was maximum when NID content reached 10%. The AA created an alkaline environment to promote solidification. In comparison to Na2SiO3 solution, NaOH solution is more effective in the adsorption of HMs. The larger ionic radius of Pb2+ relative to Cu2+, limited HMs migration ability, thereby facilitating solidification.