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Understanding the soil phosphorus (P) cycle is a prerequisite for the sustainable management of land resources. The sequential-extraction method was used to determine P fractions in 513 soils of <i>Larix principis-rupprechtii</i> Mayr. plantations. With these data, this study applied structural equation modeling to evaluate the interaction between various soil P fractions. Quantitative analysis was conducted on the importance of different soil P pools and P transformation pathways on soil P availability in a larch plantation. Our study showed that soluble inorganic P (Pi) was directly positively affected by labile Pi, labile organic P (Po), secondary mineral P, and primary mineral P, and was directly negatively affected by moderately labile Po. Soluble Pi was not directly affected by occluded P. The primary mineral P (<i>β</i> = 0.40) had the greatest total impact on soluble Pi, followed by secondary mineral P (<i>β</i> = 0.32) and labile P (labile Pi and Po, <i>β</i> = 0.31), and then occluded P (<i>β</i> = 0.11), with the total impact of moderately labile Po being relatively small (<i>β</i> = −0.06). In summary, this study reveals the important roles of soluble Pi in P transformations and in determining overall P availability in soils, as well as the extensive effects of weathering on soil P dynamics in <i>L. principis-rupprechtii</i> plantations.