Abstract: The inlet nozzle to shell weld of PWR RPV has the characteristics of complicated geometry, while the definition of ultrasonic inspection area is different among several in-service inspection codes, and these above factors cause the inaccessible zones to show non-linearity change. In order to find the special changing rules, the parametric model of the nozzle weld was established, meanwhile the simulation method which caused by the slope feature was deeply analyzed, which help to deduce the discriminant criteria for the right-side inaccessible zones simulation. Based on the criteria, with typical parameters of the PWR RPV and ASME code, it is easier to acquire the mathematics diagram of the volume for both left-side and right-side inaccessible zones to refraction angle, with the help of lots of scattered data set by 3D simulation software. The diagram can interpret an optimization method for getting minimum inaccessible zone volume for phase array or conventional method. The research takes the advantage of increasing the chance of finding the ultrasonic defects, especially for the mathematic laws of accessibly and distribution of the phase array or conventional ultrasonic technics in same type of welds.