A new small-pore compound K4Fe4P5O20 was obtained by conventional solid-state reaction in a closed crucible. The crystal structure is constructed by Fe4P5O20 units forming chains along the c axis and elliptical eight-ring channels on the a-b plane in which K cations locate inside. Such structural characteristics seem to be quite similar to those seen in the natrolite family. However, Fe ions in K4Fe4P5O20 have trigonal-bipyramidal instead of common tetrahedral coordination. Furthermore, our experimental results combined from magnetic susceptibility and Fe-57 Mossbauer spectrum measurements show mixed valence Fe3+/Fe2+ in the titled material. To the best of our knowledge, this is the first example that contains mixed valence iron ions in a so-called natrolite framework.