Abstract: X-ray excited photodynamic therapy (X-PDT) is the bravo answer of photodynamic therapy (PDT) for deep-seated tumors, as it employs X-ray as the irradiation source to overcome the limitation of light penetration depth. However, high X-ray irradiation dose caused organ lesions and side effects became the major barrier to X-PDT application. To address this issue, this work employed a classical co-precipitation reaction to synthesize NaLuF₄:15%Tb³⁺ (NLF) with an average particle size of (23.48 ± 0.91) nm, which was then coupled with the photosensitizer merocyanine 540 (MC540) to form the X-PDT system NLF–MC540 with high production of singlet oxygen. The system could induce antitumor efficacy to about 24% in relative low dose X-ray irradiation range (0.1–0.3 Gy). In vivo, when NLF–MC540 irradiated by 0.1 Gy X-ray, the tumor inhibition percentage reached 89.5% ± 5.7%. The therapeutic mechanism of low dose X-PDT was found. A significant increase of neutrophils in serum was found on the third day after X-PDT. By immunohistochemical staining of tumor sections, the Ly6G⁺, CD8⁺, and CD11c⁺ cells infiltrated in the tumor microenvironment were studied. Utilizing the bilateral tumor model, the NLF–MC540 with 0.1 Gy X-ray irradiation could inhibit both the primary tumor and the distant tumor growth. Detected by enzyme linked immunosorbent assay (ELISA), two cytokines IFN-γ and TNF-α in serum were upregulated 7 and 6 times than negative control, respectively. Detected by enzyme linked immune spot assay (ELISPOT), the number of immune cells attributable to the IFN-γ and TNF-α levels in the group of low dose X-PDT were 14 and 6 times greater than that in the negative control group, respectively. Thus, it conclude that low dose X-PDT system could successfully upregulate the levels of immune cells, stimulate the secretion of cytokines (especially IFN-γ and TNF-α), activate antitumor immunity, and finally inhibit colon tumor growth.