The size of the coarse water in steam turbine is large but the number is small.And coarse water is the main factor which causes blade water erosion in steam turbine and also leads to loss in turbine stage efficiency.Among the methods of measuring coarse water,image method has the advantage of simultaneous measurement of the parameters of particle size,speed,direction and humidity.So,in the study the image probe was adopted to measure coarse water.The measurement volume was determined by the parameters of lens,moreover a new image method was proposed to find whether coarse water is in the measure area or not according to the blur degree and the calculated particle size,speed and humidity of coarse water in the measurement volume.And then the relationships between the blur degree of particle image and the particle parameters were obtained by experimental study.A wet steam simulation experiment was carried out to calculate the spray droplet size and volume concentration which verifies the effectiveness of the measurement.The study provides a new idea for coarse water measurement.