The present invention relates to a lithium tantalate film infrared detector and the preparing method thereof. The invention comprises the following components: an infrared filtering window, a resonance chopped wave modulator, a focusing lens, a pyroelectric lithium tantalate film infrared measuring probe, a heat sink cavity, a preamplifier, a low-pass filter, an electric power and signal output interface, a casing and an environment temperature detecting filter. The method for preparing the detector comprises the following steps: selecting a substrate, growing a SiO2 layer on the front and back surface, depositing Si3N4 layer on the front surface in sequence, sputtering a Ti layer, sputtering a photoetching Pt electrode layer, producing a lithium tantalate film layer, depositing a phototeching Al electrode layer, growing a photoetching SiNX antireflection layer and sputtering a black layer to complete the double unit structure infrared probe. The focusing lens is coated with an anti-reflection film. The infrared window is pasted with a narrowband filtering film. The preamplifier is built with a JFET tube or operational amplifier. The low-pass filter is built with an operational amplifier. The resonance chopped wave modulator is driven by piezoelectricity or electromagnet, and the resonance frequency is 1Hz-1000Hz.