An impregnation method to prepare reagent indicator papers with silver triangular nanoplates (TNPs) is developed. The conditions for obtaining modified papers (type of paper, method for applying TNPs, and drying conditions) allowing for an acceptably uniform distribution of the TNPs are chosen. The spectral characteristics of the prepared materials are studied. The presence of an intense TNP surface plasmon resonance band in the diffuse reflectance spectra of the modified papers, which is linearly related to the content of the TNPs in the samples, as well as the lability of its position and shape in the presence of some substances, gives rise to the prospects of using the prepared materials as test-tools in chemical analysis.