By adopting a trilacunary [A-α-GeW9O34]10− as a precursor under hydrothermal conditions, a series of novel 4s–4f cluster substituted polyoxometalates (POMs) containing two types of in situ generated lacunary POM fragments have been made. These compounds were characterized by single-crystal X-ray diffractions, powder X-ray diffractions, infrared spectroscopy, thermogravimetric analyses, respectively. Structural analyses indicate that these compounds constructed from a seven-nuclearity K-O-Ln (Ln = Dy, Ho, Er) heterometallic cluster stabilized by mixed dilacunary Keggin [α-GeW10O37]10−/multilacunary open Wells-Dawson [Ge2W18O66]16− fragments. Furthermore, the solid luminescent properties of the compounds at room temperature were studied.