请教：ANSYS中，MASS21中三个方向的质量如何定义？-ANSYS中mass21质量单元三个方向的质量是什么意思？

没有正负之分，ansys对这个没有要求的。

为什么要加重力载荷？用mass21肯定是有影响的

看hlep文件。首先确定你的单元keyopt3
KEYOPT(3)
Rotary inertia options:
0 -- 3-D mass with rotary inertia
2 -- 3-D mass without rotary inertia
3 -- 2-D mass with rotary inertia
4 -- 2-D mass without rotary inertia

然后确定实常数格式。
Real Constants
MASSX, MASSY, MASSZ, IXX, IYY, IZZ, if KEYOPT (3) = 0
MASS, if KEYOPT (3) = 2
MASS, IZZ, if KEYOPT (3) = 3
MASS, if KEYOPT (3) = 4

(MASSX, MASSY, and MASSZ are concentrated mass components in the element
coordinate directions. IXX, IYY, and IZZ are rotary inertias about the element coordinate axes. See also KEYOPT(2)).

按照你e799bee5baa6e59b9ee7ad94361的描述，keyopt=4,实常数就写
r,编号,100