Abstract: To construct a stable and structurally controllable optical-magnetic composite nanomaterials and use it in the detection of pesticide residues, the upconversion nanomaterials (UCNPs) were prepared by the high temperature solvothermal method in this work. The effects of temperature and rotational speed on UCNPs were investigated to achieve controllable preparation of UCNPs. Three kinds of multifunctional optic-magnetic composite nanomaterials with different structures were prepared by reversed-phase microemulsion method. Scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), vibrating sample magnetometer (VSM) etc. were utilized to investigate the morphology, structure etc. of multifunctional composites. Furthermore, the response of multifunctional optic-magnetic composite nanomaterials to various pesticides in citrus was studied. The experimental results showed that temperature played a decisive role in the size of UCNPs. In different structures of UCNPs-Fe₃O₄ composites, the stronger fluorescence of A-structure (the satellite structure of UCNPs material with larger particle size) was obtained. B-structure (an interweave structure with similar particle sizes of UCNPs and Fe₃O₄) had a saturation magnetization of 20.5 emu·g⁻¹ and high fluorescence intensity. C-structure (a satellite structure with larger particle size of Fe₃O₄ material) had best magnetic properties. Meanwhile, the multifunctional optic-magnetic composites had a certain response to pyrethroid pesticide, nicotine pesticide, carbamate pesticide and organophosphorus pesticide in citrus. Due to the strongest response to pyrethroid pesticide, they could be developed for the biosensor of pyrethroid pesticide. In this work, a simple and universal synthesis method was proposed to provide theoretical ideas for the synthesis of other composite materials. This material had a great application potential in drug delivery, imaging and other fields due to their good optic-magnetic properties and biocompatibility.