Persulfate (PDS) is commonly used oxidant in in-situ chemical oxidation (ISCO) due to its low cost and high stability. PDS can be activated by heat or metal oxide in the soil to produce sulfate radical, which effectively decompose pollutants. However, PDS is also subject to self-decomposition due to various factors in soil component, including soil organic matter (SOM) and even metal oxides, which reduces the oxidant efficiency. In this study, we evaluated the kinetics of ibuprofen degradation and the oxidant utilization efficiency of heat-activated persulfate in the presence of soil. We also investigated the use of tetrapolyphosphate (TPP) as an additive to inhibit self-decomposition and enhance the oxidant utilization efficiency of thermally activated PDS. Experimental results showed that in the presence of soil, ibuprofen was degraded less by heat-activated PDS compared to expectations (from 80% to 40%), and PDS was decomposed more than in the absence of soil. The addition of TPP increased the degradation rate of ibuprofen while decreasing the rate of PDS decomposition, ultimately increasing the oxidant utilization efficiency by 170%. To investigate the mechanism behind the reaction and decomposition of PDS by soil component, we evaluated the decomposition of PDS in the presence of calcined soil (SOM removed). These results showed that whether SOM was present or not, TPP enhanced the oxidant utilization efficiency of PDS. Further experiments, including ESR analysis, radical quantification, and PMSO conversion, were conducted to gain insight into the role of TPP in heat-assisted PDS activation. It was suggested that TPP chelated to the iron oxide surface and inhibited the conversion of ≡Fe(II) to ≡Fe(IV) by PDS, which is suspected to be the reaction of self-decomposition of PDS, but rather promoted the iron activation of PDS. Therefore, this study proposes the feasibility of the efficient process of thermally activated persulfate with tetrapolyphosphate additive.