"Grid Plus T Tree" multidimensional index method in memory database partitions the whole record sets to different grid cells. Every grid data unit is connected with one data barrel, which loads records, through the grid directory. Every data unit is only mapped with one data barrel. For memory database, setting up a "T Tree" with these data units according to one field, so as to set up one "T Tree" for each data barrel according to one field. Then the grid directory can be presented by a k-dimensional (the dimension of keyword) array. The field ranges of these data units are called scale. Scale is the memory range of each "T Tree". This invention successfully overcomes the shortcomings that the current index methods for memory database neither efficiently use the memory space nor suits multi-dimension. T Tree is much more efficient than K-D Tree or R Tree in using the memory space, and it has one more one-dimensional data structure during the process of grid partition, which can help store one field of the keyword as the index.