In the invention, an embodiment provides an apparatus for determining zero-o'clock, three-o'clock, six-o'clock and nine-o'clock positions in a horizontal pipeline and a working method therefor. The apparatus includes: a support surrounding the external surface of the horizontal pipeline, and a spherical level instrument fixed on the support. A plurality of original points are marked on a plane, which pass through the spherical center and a bubble, on the spherical level instrument at every 90 degrees, wherein the original points are successively distributed on the zero-o'clock, three-o'clock, six-o'clock and nine-o'clock positions in the plane anticlockwise. The plane is in parallel with a cross section of the horizontal pipeline. When the spherical level instrument is disposed horizontally, the original point at the zero-o'clock position coincides with the bubble in the spherical level instrument, while the six-o'clock position of the plane is always tangentially with the external surface of the horizontal pipeline. When the bubble in the spherical level instrument coincides with one original point in the plane, the original point is determined to be the zero-o'clock, the three-o'clock, the six-o'clock or the nine-o'clock positions. The apparatus and the method can determine the zero-o'clock, the three-o'clock, the six-o'clock and the nine-o'clock positions in the horizontal pipeline accurately and conveniently.