Pattern formation of electroconvection in liquid crystals is a peculiar and intriguing phenomenon, and it also poses an important problem in the study of optical diffraction. This work aims to explore pattern formation and related diffraction characteristics in dielectrically negative anisotropic nematic liquid crystals. Four different electroconvection patterns are observed experimentally, namely, soft square, oblique rolls, skewed varicose, and irregular patterns.The threshold voltages required for pattern generation at different temperatures are investigated. The effect of electric field on the periodicity of patterns and the inclination angle of the relative director are studied, and the corresponding diffraction characteristics of the patterns are analyzed. The experimental results show that electroconvection patterns can be altered by adjusting the electric field and temperature. The threshold voltage required for pattern generation decreases with the increase of temperature and increases with the growth of frequency. The periodicity of soft square patterns initially decreases and then increases with the increase of frequency, while the periodicity of oblique rolls decreases with the increase of frequency. When f=25~45Hz, the oblique rolls exhibit an approximately 44° deviation from the initial director. This controllable electroconvection pattern provides a feasible solution for the realization of tunable optical diffraction devices based on nematic liquid crystals.