Here we report a fluorescent magnesium coordination polymer (Mg-CP), namely, [CH3-dpb](2)[Mg-3(1,4-NDC)(4)(mu-H2O)(2)(CH3OH)(H2O)]center dot 1.5H(2)O (1, 1,4-H2NDC = 1,4-naphthalene dicarboxylic acid, dpb = 1,4-bis(pyrid-4-yl)benzene). Compound 1 possesses a three-dimensional (3D) host-guest structure constructed by the 1,4-NDC linkers bridging the linear trinuclear secondary building units of [Mg-3(COO)(8)(mu-H2O)(2)]. The dpb molecules were in situ reacted with CH3OH resulting in photochromic cations of [CH3-dpb](+) that acted as guests located in the channels parallel to the b-axis. Photoluminescence (PL) studies indicated that 1 showed a strong green emission demonstrating sensitive fluorescence sensing of Fe3+/Cr3+ metal ions and nitro-explosive compounds. Compound 1 represents the first PL Mg-CP as a fluorescent probe for detecting metal ions. Moreover, because of the in situ encapsulation of photochromic [CH3-dpb](+) guests, 1 exhibited reversible photochromic behavior.