Characterized by their unique electrical, mechanical, and photonic properties, carbon nanotubes (CNTs) have generated a high level of research interest. Based on the study of methane coaxial jet diffusion flame, the effect of sampling substrate, sampling time and sampling height on the catalytic synthesis of CNTs was studied. The morphology and structure of carbon nanomaterials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Raman spectroscopy. The interaction between the catalyst and the substrate was analyzed with nickel nitrate as catalyst precursor and copper, nickel-chromium and nickel foam as supported substrates, respectively. The experimental results showed that sampling time and sampling height had an important influence on CNTs. Copper and nickel-chromium substrates provide open space for the synthesis of CNTs, resulting in more impurities. The unique voids of nickel foam provide excellent growth space for CNTs. SEM showed that the multi-walled CNTs were dense and uniform. TEM displayed that the catalyst particles were coated inside the CNTs. Raman spectroscopy indicated that the CNTs synthesized on copper substrates had the highest crystallinity and purity, followed by nickel-chromium and nickel foam substrates. The interaction between the catalyst and the substrate remarkably affected the growth mechanism and morphology of CNTs. The flame-synthesizing CNTs are based on the tip growth mode and the vapor-liquid-solid growth mechanism. Flame synthesis holds great potential for the cost-effective production of CNTs.