A low-back-clearance robot speed reducer includes a wheel holder (16), a pin gear shell (1), a first cycloidal wheel (6), a roller pin (8), a second cycloidal wheel (11), a gland (2), an elastic member (9), a first cross-shaped slider (5), a second cross-shaped slider (13) and a crankshaft (10). The first cycloidal wheel (6) and the second cycloidal wheel (11) are in a phase relation of 180 degrees and have a multi-point engagement transmission relationship with the pin gear shell (1) and the roller pin (8). The first cross-shaped slider (5) and the second cross-shaped slider (13) form a double cross-shaped slider structure, in which the groove profile of the sliders is of a trapezoidal groove structure. A clearance is automatically axially compensated by the elastic member (9). The center of rotation of the crankshaft (10) is coaxial with the wheel holder (16) and the gland (2).