The composition of Machangjing alkali-rich porphyry was analyzed and the data obtained are reported in this paper. The results show that the porphyry has characteristics of potassium-high calc-alkali series or shoshonite series, with SiO2 being 61.56％～71.63％, averaging 67.30％(≥56%); Al2O3 13.38%～17.18％, averaging 15.44% (≥15%); K2O 3.36％～8.92％, averaging 5.35％; K2O+Na2O 7.75％～11.55％, averaging 9.08％; K2O/Na2O 0.65～4.00, averaging 1.49; and MgO 0.40％～4.59％, averaging 1.11％. The R1-R2 diagram suggests that the porphyritic magma was formed during the late orogeny and the s yn-collision. Geochemically, the porphyry is relatively depleted in such HFSE as Nb, Ta, Ti and rich in Sr(337×10-6～718×10-6), with Y being 6.2×10-6～15.8×10-6 (≤18×10-6) and Yb being 0.20×10-6～1.63×10-6 (≤1.9×10-6). The REE have experienced strong differentiation and are thus markedly enriched in LREE. LREE/HREE ratios vary from 8.02～24.01, Sr/Y from 40.5～57.4, averaging 48.2(＞40 ), La/Yb from 17.5to 105.8, averaging 43.4(＞20), Sc from 2.5×10-6 to 2.5×10-6 (＜10×10-6), andδEu from 0.81 to 1.38. All these data indicate that the alkali-rich porphyry is geochemically characterized by adakite and belongs to C-type potassic adakite. The Machangjing porphyry is a kind of adakite and geochemically and envoirnmentally similar to the Yulong porphyry within the same ore-forming belt, which is somewhat of significance to geological prospecting.