The perovskite-type oxynitride with AB(O,N)₃ formula is a new type of functional ceramic materials, which have unique dielectric/magnetic/photocatalytic properties and prospective applications in the field of energy storage and conversion. However, the traditional preparation process takes a long time and the product purity is low. In this study, SrTa(O,N)₃ ceramic powder was synthesized and densified by a pressureless spark plasma sintering equipment with urea as nitrogen source and metal oxides as precursors. Effects of heating rate and synthesis temperature on the composition and microstructure of the powder were deeply investigated, and the dielectric properties of the optimized ceramic bulks were characterized. The results show that higher heating rate and moderate synthesis temperature are beneficial to sufficient nitridation, while the SrTa(O,N)₃ powder prepared at 100 ℃/min and 1000 ℃ possesses the highest purity (~97% oxynitride phase content) with a particle size distribution of 100-300 nm. Elements of Sr, Ta, O and N are evenly distributed. The optimized densification process is firstly sintering at 1300 ℃, with heating rate of 300 ℃/min, and dwelled for 1 min. After sintering, the density of SrTa(O,N)₃ ceramic pellet can reach >94% with a high purity. Dielectric constant and loss tangent of the material are 8349 and 10^-4 level at 300 Hz, respectively, which are superior to that reported in the literature. The high dielectric constant prepared in this study is closely related to standard density and purity, because the existence of pores and impurities can reduce the dielectric constant of materials. Therefore, high density and purity are the key factors to obtain excellent dielectric properties of SrTa(O,N)₃ oxynitride ceramics.