Provided is a method that achieves a large increase in efficiency with respect to the prescription analysis for a large granular agrochemical preparation that floats on the surface of water by establishing a scheme that makes it possible to predict the drying properties and quality of the agrochemical preparation from the prescription composition thereof without actually performing test production of the agrochemical preparation, said method being used to obtain a high-quality large granular agrochemical preparation that floats on the surface of water and that has superior production properties. The large granular agrochemical preparation that floats on the surface of water contains an active agrochemical component, a floating carrier, and water within a range of 0.3-3 mass%. The large granular agrochemical preparation that floats on the surface of water has a particle distribution that comprises 80% or more of particles having a particle size of 3 mm or more, has the property of floating on the surface of water, and breaks down at the surface of water within thirty minutes of being added to water. With regard to the total (n) types of starting materials that are all of the active agrochemical component and the auxiliary components (excluding water) that constitute the agrochemical preparation, the value (X) that is defined by the formula indicated below (wherein W1, W2, […], and Wn represent the parts by weight of each of a raw material (1), a raw material (2), […], and a raw material (n) with respect to 100 parts by weight excluding water of the large granular agrochemical preparation that floats on the surface of water, and EM1, EM2, […], and EMn represent the mass% values of the water content of each of the raw material (1), the raw material (2), […], and the raw material (n) as measured by the Karl Fischer method when left for one week in a relative humidity of 21% at 60 °C) is within the range of 0.5-2. Also provided is a production method for the large granul
