Multiphoton ionization of formaldehyde hydrated clusters is studied by time-of-flight mass spectrometry using 5 ns, 355 nm Nd: YAG laser beam with a power intensity of 10111012 W/cm2. The main products including protonated formaldehyde cluster series (CH2O)nH+(n=1-4), deprotonated formaldehyde clusters series (CH2O)nCHO+ (n=1-3, and two series based on molecules originating from H2CO ( the deprotonated and protonated forms ), H3CO+(H2O)n(n=1, 3, 5) and HCO+(H2O)n(n=1,3,5) are observed, and the simple structures are gave for some clusters. The transformation of formaldehyde mass peak is studied in different laser power density conditions. We find the ion peaks of formaldehyde monomer and water molecle ar a laser intensity of about 9.3 1011 W/cm2. Those mass peaks exist in envelope form and cannot be resolved under our experiment condition. The simple dynamic plasma sheath accelerating model is proposed to explain the physical mechanism of the envelope phenomenon.